1 /* SPDX-License-Identifier: GPL-2.0+ */
3 * Task-based RCU implementations.
5 * Copyright (C) 2020 Paul E. McKenney
8 #ifdef CONFIG_TASKS_RCU_GENERIC
9 #include "rcu_segcblist.h"
11 ////////////////////////////////////////////////////////////////////////
13 // Generic data structures.
16 typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
17 typedef void (*pregp_func_t)(struct list_head *hop);
18 typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
19 typedef void (*postscan_func_t)(struct list_head *hop);
20 typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
21 typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
24 * struct rcu_tasks_percpu - Per-CPU component of definition for a Tasks-RCU-like mechanism.
25 * @cblist: Callback list.
26 * @lock: Lock protecting per-CPU callback list.
27 * @rtp_jiffies: Jiffies counter value for statistics.
28 * @lazy_timer: Timer to unlazify callbacks.
29 * @urgent_gp: Number of additional non-lazy grace periods.
30 * @rtp_n_lock_retries: Rough lock-contention statistic.
31 * @rtp_work: Work queue for invoking callbacks.
32 * @rtp_irq_work: IRQ work queue for deferred wakeups.
33 * @barrier_q_head: RCU callback for barrier operation.
34 * @rtp_blkd_tasks: List of tasks blocked as readers.
35 * @cpu: CPU number corresponding to this entry.
36 * @rtpp: Pointer to the rcu_tasks structure.
38 struct rcu_tasks_percpu {
39 struct rcu_segcblist cblist;
40 raw_spinlock_t __private lock;
41 unsigned long rtp_jiffies;
42 unsigned long rtp_n_lock_retries;
43 struct timer_list lazy_timer;
44 unsigned int urgent_gp;
45 struct work_struct rtp_work;
46 struct irq_work rtp_irq_work;
47 struct rcu_head barrier_q_head;
48 struct list_head rtp_blkd_tasks;
50 struct rcu_tasks *rtpp;
54 * struct rcu_tasks - Definition for a Tasks-RCU-like mechanism.
55 * @cbs_wait: RCU wait allowing a new callback to get kthread's attention.
56 * @cbs_gbl_lock: Lock protecting callback list.
57 * @tasks_gp_mutex: Mutex protecting grace period, needed during mid-boot dead zone.
58 * @gp_func: This flavor's grace-period-wait function.
59 * @gp_state: Grace period's most recent state transition (debugging).
60 * @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.
61 * @init_fract: Initial backoff sleep interval.
62 * @gp_jiffies: Time of last @gp_state transition.
63 * @gp_start: Most recent grace-period start in jiffies.
64 * @tasks_gp_seq: Number of grace periods completed since boot.
65 * @n_ipis: Number of IPIs sent to encourage grace periods to end.
66 * @n_ipis_fails: Number of IPI-send failures.
67 * @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
68 * @lazy_jiffies: Number of jiffies to allow callbacks to be lazy.
69 * @pregp_func: This flavor's pre-grace-period function (optional).
70 * @pertask_func: This flavor's per-task scan function (optional).
71 * @postscan_func: This flavor's post-task scan function (optional).
72 * @holdouts_func: This flavor's holdout-list scan function (optional).
73 * @postgp_func: This flavor's post-grace-period function (optional).
74 * @call_func: This flavor's call_rcu()-equivalent function.
75 * @rtpcpu: This flavor's rcu_tasks_percpu structure.
76 * @percpu_enqueue_shift: Shift down CPU ID this much when enqueuing callbacks.
77 * @percpu_enqueue_lim: Number of per-CPU callback queues in use for enqueuing.
78 * @percpu_dequeue_lim: Number of per-CPU callback queues in use for dequeuing.
79 * @percpu_dequeue_gpseq: RCU grace-period number to propagate enqueue limit to dequeuers.
80 * @barrier_q_mutex: Serialize barrier operations.
81 * @barrier_q_count: Number of queues being waited on.
82 * @barrier_q_completion: Barrier wait/wakeup mechanism.
83 * @barrier_q_seq: Sequence number for barrier operations.
84 * @name: This flavor's textual name.
85 * @kname: This flavor's kthread name.
88 struct rcuwait cbs_wait;
89 raw_spinlock_t cbs_gbl_lock;
90 struct mutex tasks_gp_mutex;
94 unsigned long gp_jiffies;
95 unsigned long gp_start;
96 unsigned long tasks_gp_seq;
98 unsigned long n_ipis_fails;
99 struct task_struct *kthread_ptr;
100 unsigned long lazy_jiffies;
101 rcu_tasks_gp_func_t gp_func;
102 pregp_func_t pregp_func;
103 pertask_func_t pertask_func;
104 postscan_func_t postscan_func;
105 holdouts_func_t holdouts_func;
106 postgp_func_t postgp_func;
107 call_rcu_func_t call_func;
108 struct rcu_tasks_percpu __percpu *rtpcpu;
109 int percpu_enqueue_shift;
110 int percpu_enqueue_lim;
111 int percpu_dequeue_lim;
112 unsigned long percpu_dequeue_gpseq;
113 struct mutex barrier_q_mutex;
114 atomic_t barrier_q_count;
115 struct completion barrier_q_completion;
116 unsigned long barrier_q_seq;
121 static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
123 #define DEFINE_RCU_TASKS(rt_name, gp, call, n) \
124 static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = { \
125 .lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ## __percpu.cbs_pcpu_lock), \
126 .rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup), \
128 static struct rcu_tasks rt_name = \
130 .cbs_wait = __RCUWAIT_INITIALIZER(rt_name.wait), \
131 .cbs_gbl_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_gbl_lock), \
132 .tasks_gp_mutex = __MUTEX_INITIALIZER(rt_name.tasks_gp_mutex), \
135 .rtpcpu = &rt_name ## __percpu, \
136 .lazy_jiffies = DIV_ROUND_UP(HZ, 4), \
138 .percpu_enqueue_shift = order_base_2(CONFIG_NR_CPUS), \
139 .percpu_enqueue_lim = 1, \
140 .percpu_dequeue_lim = 1, \
141 .barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex), \
142 .barrier_q_seq = (0UL - 50UL) << RCU_SEQ_CTR_SHIFT, \
146 #ifdef CONFIG_TASKS_RCU
147 /* Track exiting tasks in order to allow them to be waited for. */
148 DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
150 /* Report delay in synchronize_srcu() completion in rcu_tasks_postscan(). */
151 static void tasks_rcu_exit_srcu_stall(struct timer_list *unused);
152 static DEFINE_TIMER(tasks_rcu_exit_srcu_stall_timer, tasks_rcu_exit_srcu_stall);
155 /* Avoid IPIing CPUs early in the grace period. */
156 #define RCU_TASK_IPI_DELAY (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) ? HZ / 2 : 0)
157 static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
158 module_param(rcu_task_ipi_delay, int, 0644);
160 /* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
161 #define RCU_TASK_BOOT_STALL_TIMEOUT (HZ * 30)
162 #define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
163 static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
164 module_param(rcu_task_stall_timeout, int, 0644);
165 #define RCU_TASK_STALL_INFO (HZ * 10)
166 static int rcu_task_stall_info __read_mostly = RCU_TASK_STALL_INFO;
167 module_param(rcu_task_stall_info, int, 0644);
168 static int rcu_task_stall_info_mult __read_mostly = 3;
169 module_param(rcu_task_stall_info_mult, int, 0444);
171 static int rcu_task_enqueue_lim __read_mostly = -1;
172 module_param(rcu_task_enqueue_lim, int, 0444);
174 static bool rcu_task_cb_adjust;
175 static int rcu_task_contend_lim __read_mostly = 100;
176 module_param(rcu_task_contend_lim, int, 0444);
177 static int rcu_task_collapse_lim __read_mostly = 10;
178 module_param(rcu_task_collapse_lim, int, 0444);
179 static int rcu_task_lazy_lim __read_mostly = 32;
180 module_param(rcu_task_lazy_lim, int, 0444);
182 /* RCU tasks grace-period state for debugging. */
184 #define RTGS_WAIT_WAIT_CBS 1
185 #define RTGS_WAIT_GP 2
186 #define RTGS_PRE_WAIT_GP 3
187 #define RTGS_SCAN_TASKLIST 4
188 #define RTGS_POST_SCAN_TASKLIST 5
189 #define RTGS_WAIT_SCAN_HOLDOUTS 6
190 #define RTGS_SCAN_HOLDOUTS 7
191 #define RTGS_POST_GP 8
192 #define RTGS_WAIT_READERS 9
193 #define RTGS_INVOKE_CBS 10
194 #define RTGS_WAIT_CBS 11
195 #ifndef CONFIG_TINY_RCU
196 static const char * const rcu_tasks_gp_state_names[] = {
198 "RTGS_WAIT_WAIT_CBS",
201 "RTGS_SCAN_TASKLIST",
202 "RTGS_POST_SCAN_TASKLIST",
203 "RTGS_WAIT_SCAN_HOLDOUTS",
204 "RTGS_SCAN_HOLDOUTS",
210 #endif /* #ifndef CONFIG_TINY_RCU */
212 ////////////////////////////////////////////////////////////////////////
216 static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp);
218 /* Record grace-period phase and time. */
219 static void set_tasks_gp_state(struct rcu_tasks *rtp, int newstate)
221 rtp->gp_state = newstate;
222 rtp->gp_jiffies = jiffies;
225 #ifndef CONFIG_TINY_RCU
226 /* Return state name. */
227 static const char *tasks_gp_state_getname(struct rcu_tasks *rtp)
229 int i = data_race(rtp->gp_state); // Let KCSAN detect update races
230 int j = READ_ONCE(i); // Prevent the compiler from reading twice
232 if (j >= ARRAY_SIZE(rcu_tasks_gp_state_names))
234 return rcu_tasks_gp_state_names[j];
236 #endif /* #ifndef CONFIG_TINY_RCU */
238 // Initialize per-CPU callback lists for the specified flavor of
239 // Tasks RCU. Do not enqueue callbacks before this function is invoked.
240 static void cblist_init_generic(struct rcu_tasks *rtp)
247 if (rcu_task_enqueue_lim < 0) {
248 rcu_task_enqueue_lim = 1;
249 rcu_task_cb_adjust = true;
250 } else if (rcu_task_enqueue_lim == 0) {
251 rcu_task_enqueue_lim = 1;
253 lim = rcu_task_enqueue_lim;
255 if (lim > nr_cpu_ids)
257 shift = ilog2(nr_cpu_ids / lim);
258 if (((nr_cpu_ids - 1) >> shift) >= lim)
260 WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
261 WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
262 smp_store_release(&rtp->percpu_enqueue_lim, lim);
263 for_each_possible_cpu(cpu) {
264 struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
266 WARN_ON_ONCE(!rtpcp);
268 raw_spin_lock_init(&ACCESS_PRIVATE(rtpcp, lock));
269 local_irq_save(flags); // serialize initialization
270 if (rcu_segcblist_empty(&rtpcp->cblist))
271 rcu_segcblist_init(&rtpcp->cblist);
272 local_irq_restore(flags);
273 INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
276 if (!rtpcp->rtp_blkd_tasks.next)
277 INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
280 pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d.\n", rtp->name,
281 data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim), rcu_task_cb_adjust);
284 // Compute wakeup time for lazy callback timer.
285 static unsigned long rcu_tasks_lazy_time(struct rcu_tasks *rtp)
287 return jiffies + rtp->lazy_jiffies;
290 // Timer handler that unlazifies lazy callbacks.
291 static void call_rcu_tasks_generic_timer(struct timer_list *tlp)
294 bool needwake = false;
295 struct rcu_tasks *rtp;
296 struct rcu_tasks_percpu *rtpcp = from_timer(rtpcp, tlp, lazy_timer);
299 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
300 if (!rcu_segcblist_empty(&rtpcp->cblist) && rtp->lazy_jiffies) {
301 if (!rtpcp->urgent_gp)
302 rtpcp->urgent_gp = 1;
304 mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
306 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
308 rcuwait_wake_up(&rtp->cbs_wait);
311 // IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
312 static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
314 struct rcu_tasks *rtp;
315 struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct rcu_tasks_percpu, rtp_irq_work);
318 rcuwait_wake_up(&rtp->cbs_wait);
321 // Enqueue a callback for the specified flavor of Tasks RCU.
322 static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
323 struct rcu_tasks *rtp)
327 bool havekthread = smp_load_acquire(&rtp->kthread_ptr);
330 bool needadjust = false;
332 struct rcu_tasks_percpu *rtpcp;
336 local_irq_save(flags);
338 ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
339 chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_possible_mask);
340 rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
341 if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
342 raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
344 if (rtpcp->rtp_jiffies != j) {
345 rtpcp->rtp_jiffies = j;
346 rtpcp->rtp_n_lock_retries = 0;
348 if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
349 READ_ONCE(rtp->percpu_enqueue_lim) != nr_cpu_ids)
350 needadjust = true; // Defer adjustment to avoid deadlock.
352 // Queuing callbacks before initialization not yet supported.
353 if (WARN_ON_ONCE(!rcu_segcblist_is_enabled(&rtpcp->cblist)))
354 rcu_segcblist_init(&rtpcp->cblist);
355 needwake = (func == wakeme_after_rcu) ||
356 (rcu_segcblist_n_cbs(&rtpcp->cblist) == rcu_task_lazy_lim);
357 if (havekthread && !needwake && !timer_pending(&rtpcp->lazy_timer)) {
358 if (rtp->lazy_jiffies)
359 mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
361 needwake = rcu_segcblist_empty(&rtpcp->cblist);
364 rtpcp->urgent_gp = 3;
365 rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
366 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
367 if (unlikely(needadjust)) {
368 raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
369 if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
370 WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
371 WRITE_ONCE(rtp->percpu_dequeue_lim, nr_cpu_ids);
372 smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
373 pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
375 raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
378 /* We can't create the thread unless interrupts are enabled. */
379 if (needwake && READ_ONCE(rtp->kthread_ptr))
380 irq_work_queue(&rtpcp->rtp_irq_work);
383 // RCU callback function for rcu_barrier_tasks_generic().
384 static void rcu_barrier_tasks_generic_cb(struct rcu_head *rhp)
386 struct rcu_tasks *rtp;
387 struct rcu_tasks_percpu *rtpcp;
389 rtpcp = container_of(rhp, struct rcu_tasks_percpu, barrier_q_head);
391 if (atomic_dec_and_test(&rtp->barrier_q_count))
392 complete(&rtp->barrier_q_completion);
395 // Wait for all in-flight callbacks for the specified RCU Tasks flavor.
396 // Operates in a manner similar to rcu_barrier().
397 static void rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
401 struct rcu_tasks_percpu *rtpcp;
402 unsigned long s = rcu_seq_snap(&rtp->barrier_q_seq);
404 mutex_lock(&rtp->barrier_q_mutex);
405 if (rcu_seq_done(&rtp->barrier_q_seq, s)) {
407 mutex_unlock(&rtp->barrier_q_mutex);
410 rcu_seq_start(&rtp->barrier_q_seq);
411 init_completion(&rtp->barrier_q_completion);
412 atomic_set(&rtp->barrier_q_count, 2);
413 for_each_possible_cpu(cpu) {
414 if (cpu >= smp_load_acquire(&rtp->percpu_dequeue_lim))
416 rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
417 rtpcp->barrier_q_head.func = rcu_barrier_tasks_generic_cb;
418 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
419 if (rcu_segcblist_entrain(&rtpcp->cblist, &rtpcp->barrier_q_head))
420 atomic_inc(&rtp->barrier_q_count);
421 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
423 if (atomic_sub_and_test(2, &rtp->barrier_q_count))
424 complete(&rtp->barrier_q_completion);
425 wait_for_completion(&rtp->barrier_q_completion);
426 rcu_seq_end(&rtp->barrier_q_seq);
427 mutex_unlock(&rtp->barrier_q_mutex);
430 // Advance callbacks and indicate whether either a grace period or
431 // callback invocation is needed.
432 static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
437 bool gpdone = poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
443 dequeue_limit = smp_load_acquire(&rtp->percpu_dequeue_lim);
444 for (cpu = 0; cpu < dequeue_limit; cpu++) {
445 struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
447 /* Advance and accelerate any new callbacks. */
448 if (!rcu_segcblist_n_cbs(&rtpcp->cblist))
450 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
451 // Should we shrink down to a single callback queue?
452 n = rcu_segcblist_n_cbs(&rtpcp->cblist);
458 rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
459 (void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
460 if (rtpcp->urgent_gp > 0 && rcu_segcblist_pend_cbs(&rtpcp->cblist)) {
461 if (rtp->lazy_jiffies)
464 } else if (rcu_segcblist_empty(&rtpcp->cblist)) {
465 rtpcp->urgent_gp = 0;
467 if (rcu_segcblist_ready_cbs(&rtpcp->cblist))
469 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
472 // Shrink down to a single callback queue if appropriate.
473 // This is done in two stages: (1) If there are no more than
474 // rcu_task_collapse_lim callbacks on CPU 0 and none on any other
475 // CPU, limit enqueueing to CPU 0. (2) After an RCU grace period,
476 // if there has not been an increase in callbacks, limit dequeuing
477 // to CPU 0. Note the matching RCU read-side critical section in
478 // call_rcu_tasks_generic().
479 if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
480 raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
481 if (rtp->percpu_enqueue_lim > 1) {
482 WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
483 smp_store_release(&rtp->percpu_enqueue_lim, 1);
484 rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
486 pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
488 raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
490 if (rcu_task_cb_adjust && !ncbsnz && gpdone) {
491 raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
492 if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
493 WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
494 pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
496 if (rtp->percpu_dequeue_lim == 1) {
497 for (cpu = rtp->percpu_dequeue_lim; cpu < nr_cpu_ids; cpu++) {
498 struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
500 WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
503 raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
509 // Advance callbacks and invoke any that are ready.
510 static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu *rtpcp)
517 struct rcu_head *rhp;
518 struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
519 struct rcu_tasks_percpu *rtpcp_next;
522 cpunext = cpu * 2 + 1;
523 if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
524 rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
525 cpuwq = rcu_cpu_beenfullyonline(cpunext) ? cpunext : WORK_CPU_UNBOUND;
526 queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
528 if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
529 rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
530 cpuwq = rcu_cpu_beenfullyonline(cpunext) ? cpunext : WORK_CPU_UNBOUND;
531 queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
535 if (rcu_segcblist_empty(&rtpcp->cblist) || !cpu_possible(cpu))
537 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
538 rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
539 rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);
540 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
542 for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
543 debug_rcu_head_callback(rhp);
549 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
550 rcu_segcblist_add_len(&rtpcp->cblist, -len);
551 (void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
552 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
555 // Workqueue flood to advance callbacks and invoke any that are ready.
556 static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp)
558 struct rcu_tasks *rtp;
559 struct rcu_tasks_percpu *rtpcp = container_of(wp, struct rcu_tasks_percpu, rtp_work);
562 rcu_tasks_invoke_cbs(rtp, rtpcp);
565 // Wait for one grace period.
566 static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot)
570 mutex_lock(&rtp->tasks_gp_mutex);
572 // If there were none, wait a bit and start over.
573 if (unlikely(midboot)) {
576 mutex_unlock(&rtp->tasks_gp_mutex);
577 set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
578 rcuwait_wait_event(&rtp->cbs_wait,
579 (needgpcb = rcu_tasks_need_gpcb(rtp)),
581 mutex_lock(&rtp->tasks_gp_mutex);
584 if (needgpcb & 0x2) {
585 // Wait for one grace period.
586 set_tasks_gp_state(rtp, RTGS_WAIT_GP);
587 rtp->gp_start = jiffies;
588 rcu_seq_start(&rtp->tasks_gp_seq);
590 rcu_seq_end(&rtp->tasks_gp_seq);
594 set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
595 rcu_tasks_invoke_cbs(rtp, per_cpu_ptr(rtp->rtpcpu, 0));
596 mutex_unlock(&rtp->tasks_gp_mutex);
599 // RCU-tasks kthread that detects grace periods and invokes callbacks.
600 static int __noreturn rcu_tasks_kthread(void *arg)
603 struct rcu_tasks *rtp = arg;
605 for_each_possible_cpu(cpu) {
606 struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
608 timer_setup(&rtpcp->lazy_timer, call_rcu_tasks_generic_timer, 0);
609 rtpcp->urgent_gp = 1;
612 /* Run on housekeeping CPUs by default. Sysadm can move if desired. */
613 housekeeping_affine(current, HK_TYPE_RCU);
614 smp_store_release(&rtp->kthread_ptr, current); // Let GPs start!
617 * Each pass through the following loop makes one check for
618 * newly arrived callbacks, and, if there are some, waits for
619 * one RCU-tasks grace period and then invokes the callbacks.
620 * This loop is terminated by the system going down. ;-)
623 // Wait for one grace period and invoke any callbacks
625 rcu_tasks_one_gp(rtp, false);
627 // Paranoid sleep to keep this from entering a tight loop.
628 schedule_timeout_idle(rtp->gp_sleep);
632 // Wait for a grace period for the specified flavor of Tasks RCU.
633 static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
635 /* Complain if the scheduler has not started. */
636 if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
637 "synchronize_%s() called too soon", rtp->name))
640 // If the grace-period kthread is running, use it.
641 if (READ_ONCE(rtp->kthread_ptr)) {
642 wait_rcu_gp(rtp->call_func);
645 rcu_tasks_one_gp(rtp, true);
648 /* Spawn RCU-tasks grace-period kthread. */
649 static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
651 struct task_struct *t;
653 t = kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname);
654 if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread, OOM is now expected behavior\n", __func__, rtp->name))
656 smp_mb(); /* Ensure others see full kthread. */
659 #ifndef CONFIG_TINY_RCU
662 * Print any non-default Tasks RCU settings.
664 static void __init rcu_tasks_bootup_oddness(void)
666 #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
669 if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
670 pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
671 rtsimc = clamp(rcu_task_stall_info_mult, 1, 10);
672 if (rtsimc != rcu_task_stall_info_mult) {
673 pr_info("\tTasks-RCU CPU stall info multiplier clamped to %d (rcu_task_stall_info_mult).\n", rtsimc);
674 rcu_task_stall_info_mult = rtsimc;
676 #endif /* #ifdef CONFIG_TASKS_RCU */
677 #ifdef CONFIG_TASKS_RCU
678 pr_info("\tTrampoline variant of Tasks RCU enabled.\n");
679 #endif /* #ifdef CONFIG_TASKS_RCU */
680 #ifdef CONFIG_TASKS_RUDE_RCU
681 pr_info("\tRude variant of Tasks RCU enabled.\n");
682 #endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
683 #ifdef CONFIG_TASKS_TRACE_RCU
684 pr_info("\tTracing variant of Tasks RCU enabled.\n");
685 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
688 #endif /* #ifndef CONFIG_TINY_RCU */
690 #ifndef CONFIG_TINY_RCU
691 /* Dump out rcutorture-relevant state common to all RCU-tasks flavors. */
692 static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
695 bool havecbs = false;
696 bool haveurgent = false;
697 bool haveurgentcbs = false;
699 for_each_possible_cpu(cpu) {
700 struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
702 if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)))
704 if (data_race(rtpcp->urgent_gp))
706 if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)) && data_race(rtpcp->urgent_gp))
707 haveurgentcbs = true;
708 if (havecbs && haveurgent && haveurgentcbs)
711 pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c%c%c l:%lu %s\n",
713 tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
714 jiffies - data_race(rtp->gp_jiffies),
715 data_race(rcu_seq_current(&rtp->tasks_gp_seq)),
716 data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
717 ".k"[!!data_race(rtp->kthread_ptr)],
724 #endif // #ifndef CONFIG_TINY_RCU
726 static void exit_tasks_rcu_finish_trace(struct task_struct *t);
728 #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
730 ////////////////////////////////////////////////////////////////////////
732 // Shared code between task-list-scanning variants of Tasks RCU.
734 /* Wait for one RCU-tasks grace period. */
735 static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
737 struct task_struct *g;
741 unsigned long lastinfo;
742 unsigned long lastreport;
743 bool reported = false;
745 struct task_struct *t;
747 set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP);
748 rtp->pregp_func(&holdouts);
751 * There were callbacks, so we need to wait for an RCU-tasks
752 * grace period. Start off by scanning the task list for tasks
753 * that are not already voluntarily blocked. Mark these tasks
754 * and make a list of them in holdouts.
756 set_tasks_gp_state(rtp, RTGS_SCAN_TASKLIST);
757 if (rtp->pertask_func) {
759 for_each_process_thread(g, t)
760 rtp->pertask_func(t, &holdouts);
764 set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
765 rtp->postscan_func(&holdouts);
768 * Each pass through the following loop scans the list of holdout
769 * tasks, removing any that are no longer holdouts. When the list
770 * is empty, we are done.
772 lastreport = jiffies;
773 lastinfo = lastreport;
774 rtsi = READ_ONCE(rcu_task_stall_info);
776 // Start off with initial wait and slowly back off to 1 HZ wait.
777 fract = rtp->init_fract;
779 while (!list_empty(&holdouts)) {
785 // Slowly back off waiting for holdouts
786 set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
787 if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
788 schedule_timeout_idle(fract);
790 exp = jiffies_to_nsecs(fract);
791 __set_current_state(TASK_IDLE);
792 schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD);
798 rtst = READ_ONCE(rcu_task_stall_timeout);
799 needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
801 lastreport = jiffies;
805 WARN_ON(signal_pending(current));
806 set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS);
807 rtp->holdouts_func(&holdouts, needreport, &firstreport);
809 // Print pre-stall informational messages if needed.
811 if (rtsi > 0 && !reported && time_after(j, lastinfo + rtsi)) {
813 rtsi = rtsi * rcu_task_stall_info_mult;
814 pr_info("%s: %s grace period number %lu (since boot) is %lu jiffies old.\n",
815 __func__, rtp->kname, rtp->tasks_gp_seq, j - rtp->gp_start);
819 set_tasks_gp_state(rtp, RTGS_POST_GP);
820 rtp->postgp_func(rtp);
823 #endif /* #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU) */
825 #ifdef CONFIG_TASKS_RCU
827 ////////////////////////////////////////////////////////////////////////
829 // Simple variant of RCU whose quiescent states are voluntary context
830 // switch, cond_resched_tasks_rcu_qs(), user-space execution, and idle.
831 // As such, grace periods can take one good long time. There are no
832 // read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
833 // because this implementation is intended to get the system into a safe
834 // state for some of the manipulations involved in tracing and the like.
835 // Finally, this implementation does not support high call_rcu_tasks()
836 // rates from multiple CPUs. If this is required, per-CPU callback lists
839 // The implementation uses rcu_tasks_wait_gp(), which relies on function
840 // pointers in the rcu_tasks structure. The rcu_spawn_tasks_kthread()
841 // function sets these function pointers up so that rcu_tasks_wait_gp()
842 // invokes these functions in this order:
844 // rcu_tasks_pregp_step():
845 // Invokes synchronize_rcu() in order to wait for all in-flight
846 // t->on_rq and t->nvcsw transitions to complete. This works because
847 // all such transitions are carried out with interrupts disabled.
848 // rcu_tasks_pertask(), invoked on every non-idle task:
849 // For every runnable non-idle task other than the current one, use
850 // get_task_struct() to pin down that task, snapshot that task's
851 // number of voluntary context switches, and add that task to the
853 // rcu_tasks_postscan():
854 // Invoke synchronize_srcu() to ensure that all tasks that were
855 // in the process of exiting (and which thus might not know to
856 // synchronize with this RCU Tasks grace period) have completed
858 // check_all_holdout_tasks(), repeatedly until holdout list is empty:
859 // Scans the holdout list, attempting to identify a quiescent state
860 // for each task on the list. If there is a quiescent state, the
861 // corresponding task is removed from the holdout list.
862 // rcu_tasks_postgp():
863 // Invokes synchronize_rcu() in order to ensure that all prior
864 // t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks
865 // to have happened before the end of this RCU Tasks grace period.
866 // Again, this works because all such transitions are carried out
867 // with interrupts disabled.
869 // For each exiting task, the exit_tasks_rcu_start() and
870 // exit_tasks_rcu_finish() functions begin and end, respectively, the SRCU
871 // read-side critical sections waited for by rcu_tasks_postscan().
873 // Pre-grace-period update-side code is ordered before the grace
874 // via the raw_spin_lock.*rcu_node(). Pre-grace-period read-side code
875 // is ordered before the grace period via synchronize_rcu() call in
876 // rcu_tasks_pregp_step() and by the scheduler's locks and interrupt
879 /* Pre-grace-period preparation. */
880 static void rcu_tasks_pregp_step(struct list_head *hop)
883 * Wait for all pre-existing t->on_rq and t->nvcsw transitions
884 * to complete. Invoking synchronize_rcu() suffices because all
885 * these transitions occur with interrupts disabled. Without this
886 * synchronize_rcu(), a read-side critical section that started
887 * before the grace period might be incorrectly seen as having
888 * started after the grace period.
890 * This synchronize_rcu() also dispenses with the need for a
891 * memory barrier on the first store to t->rcu_tasks_holdout,
892 * as it forces the store to happen after the beginning of the
898 /* Check for quiescent states since the pregp's synchronize_rcu() */
899 static bool rcu_tasks_is_holdout(struct task_struct *t)
903 /* Has the task been seen voluntarily sleeping? */
904 if (!READ_ONCE(t->on_rq))
908 * Idle tasks (or idle injection) within the idle loop are RCU-tasks
909 * quiescent states. But CPU boot code performed by the idle task
910 * isn't a quiescent state.
917 /* Idle tasks on offline CPUs are RCU-tasks quiescent states. */
918 if (t == idle_task(cpu) && !rcu_cpu_online(cpu))
924 /* Per-task initial processing. */
925 static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
927 if (t != current && rcu_tasks_is_holdout(t)) {
929 t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
930 WRITE_ONCE(t->rcu_tasks_holdout, true);
931 list_add(&t->rcu_tasks_holdout_list, hop);
935 /* Processing between scanning taskslist and draining the holdout list. */
936 static void rcu_tasks_postscan(struct list_head *hop)
938 int rtsi = READ_ONCE(rcu_task_stall_info);
940 if (!IS_ENABLED(CONFIG_TINY_RCU)) {
941 tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
942 add_timer(&tasks_rcu_exit_srcu_stall_timer);
946 * Exiting tasks may escape the tasklist scan. Those are vulnerable
947 * until their final schedule() with TASK_DEAD state. To cope with
948 * this, divide the fragile exit path part in two intersecting
949 * read side critical sections:
951 * 1) An _SRCU_ read side starting before calling exit_notify(),
952 * which may remove the task from the tasklist, and ending after
953 * the final preempt_disable() call in do_exit().
955 * 2) An _RCU_ read side starting with the final preempt_disable()
956 * call in do_exit() and ending with the final call to schedule()
957 * with TASK_DEAD state.
959 * This handles the part 1). And postgp will handle part 2) with a
960 * call to synchronize_rcu().
962 synchronize_srcu(&tasks_rcu_exit_srcu);
964 if (!IS_ENABLED(CONFIG_TINY_RCU))
965 del_timer_sync(&tasks_rcu_exit_srcu_stall_timer);
968 /* See if tasks are still holding out, complain if so. */
969 static void check_holdout_task(struct task_struct *t,
970 bool needreport, bool *firstreport)
974 if (!READ_ONCE(t->rcu_tasks_holdout) ||
975 t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
976 !rcu_tasks_is_holdout(t) ||
977 (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
978 !is_idle_task(t) && READ_ONCE(t->rcu_tasks_idle_cpu) >= 0)) {
979 WRITE_ONCE(t->rcu_tasks_holdout, false);
980 list_del_init(&t->rcu_tasks_holdout_list);
984 rcu_request_urgent_qs_task(t);
988 pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
989 *firstreport = false;
992 pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
993 t, ".I"[is_idle_task(t)],
994 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
995 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
996 data_race(t->rcu_tasks_idle_cpu), cpu);
1000 /* Scan the holdout lists for tasks no longer holding out. */
1001 static void check_all_holdout_tasks(struct list_head *hop,
1002 bool needreport, bool *firstreport)
1004 struct task_struct *t, *t1;
1006 list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list) {
1007 check_holdout_task(t, needreport, firstreport);
1012 /* Finish off the Tasks-RCU grace period. */
1013 static void rcu_tasks_postgp(struct rcu_tasks *rtp)
1016 * Because ->on_rq and ->nvcsw are not guaranteed to have a full
1017 * memory barriers prior to them in the schedule() path, memory
1018 * reordering on other CPUs could cause their RCU-tasks read-side
1019 * critical sections to extend past the end of the grace period.
1020 * However, because these ->nvcsw updates are carried out with
1021 * interrupts disabled, we can use synchronize_rcu() to force the
1022 * needed ordering on all such CPUs.
1024 * This synchronize_rcu() also confines all ->rcu_tasks_holdout
1025 * accesses to be within the grace period, avoiding the need for
1026 * memory barriers for ->rcu_tasks_holdout accesses.
1028 * In addition, this synchronize_rcu() waits for exiting tasks
1029 * to complete their final preempt_disable() region of execution,
1030 * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu),
1031 * enforcing the whole region before tasklist removal until
1032 * the final schedule() with TASK_DEAD state to be an RCU TASKS
1033 * read side critical section.
1038 void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
1039 DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks");
1041 static void tasks_rcu_exit_srcu_stall(struct timer_list *unused)
1043 #ifndef CONFIG_TINY_RCU
1046 rtsi = READ_ONCE(rcu_task_stall_info);
1047 pr_info("%s: %s grace period number %lu (since boot) gp_state: %s is %lu jiffies old.\n",
1048 __func__, rcu_tasks.kname, rcu_tasks.tasks_gp_seq,
1049 tasks_gp_state_getname(&rcu_tasks), jiffies - rcu_tasks.gp_jiffies);
1050 pr_info("Please check any exiting tasks stuck between calls to exit_tasks_rcu_start() and exit_tasks_rcu_finish()\n");
1051 tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
1052 add_timer(&tasks_rcu_exit_srcu_stall_timer);
1053 #endif // #ifndef CONFIG_TINY_RCU
1057 * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
1058 * @rhp: structure to be used for queueing the RCU updates.
1059 * @func: actual callback function to be invoked after the grace period
1061 * The callback function will be invoked some time after a full grace
1062 * period elapses, in other words after all currently executing RCU
1063 * read-side critical sections have completed. call_rcu_tasks() assumes
1064 * that the read-side critical sections end at a voluntary context
1065 * switch (not a preemption!), cond_resched_tasks_rcu_qs(), entry into idle,
1066 * or transition to usermode execution. As such, there are no read-side
1067 * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
1068 * this primitive is intended to determine that all tasks have passed
1069 * through a safe state, not so much for data-structure synchronization.
1071 * See the description of call_rcu() for more detailed information on
1072 * memory ordering guarantees.
1074 void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
1076 call_rcu_tasks_generic(rhp, func, &rcu_tasks);
1078 EXPORT_SYMBOL_GPL(call_rcu_tasks);
1081 * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
1083 * Control will return to the caller some time after a full rcu-tasks
1084 * grace period has elapsed, in other words after all currently
1085 * executing rcu-tasks read-side critical sections have elapsed. These
1086 * read-side critical sections are delimited by calls to schedule(),
1087 * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
1088 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
1090 * This is a very specialized primitive, intended only for a few uses in
1091 * tracing and other situations requiring manipulation of function
1092 * preambles and profiling hooks. The synchronize_rcu_tasks() function
1093 * is not (yet) intended for heavy use from multiple CPUs.
1095 * See the description of synchronize_rcu() for more detailed information
1096 * on memory ordering guarantees.
1098 void synchronize_rcu_tasks(void)
1100 synchronize_rcu_tasks_generic(&rcu_tasks);
1102 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
1105 * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
1107 * Although the current implementation is guaranteed to wait, it is not
1108 * obligated to, for example, if there are no pending callbacks.
1110 void rcu_barrier_tasks(void)
1112 rcu_barrier_tasks_generic(&rcu_tasks);
1114 EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
1116 static int rcu_tasks_lazy_ms = -1;
1117 module_param(rcu_tasks_lazy_ms, int, 0444);
1119 static int __init rcu_spawn_tasks_kthread(void)
1121 cblist_init_generic(&rcu_tasks);
1122 rcu_tasks.gp_sleep = HZ / 10;
1123 rcu_tasks.init_fract = HZ / 10;
1124 if (rcu_tasks_lazy_ms >= 0)
1125 rcu_tasks.lazy_jiffies = msecs_to_jiffies(rcu_tasks_lazy_ms);
1126 rcu_tasks.pregp_func = rcu_tasks_pregp_step;
1127 rcu_tasks.pertask_func = rcu_tasks_pertask;
1128 rcu_tasks.postscan_func = rcu_tasks_postscan;
1129 rcu_tasks.holdouts_func = check_all_holdout_tasks;
1130 rcu_tasks.postgp_func = rcu_tasks_postgp;
1131 rcu_spawn_tasks_kthread_generic(&rcu_tasks);
1135 #if !defined(CONFIG_TINY_RCU)
1136 void show_rcu_tasks_classic_gp_kthread(void)
1138 show_rcu_tasks_generic_gp_kthread(&rcu_tasks, "");
1140 EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);
1141 #endif // !defined(CONFIG_TINY_RCU)
1143 struct task_struct *get_rcu_tasks_gp_kthread(void)
1145 return rcu_tasks.kthread_ptr;
1147 EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);
1150 * Contribute to protect against tasklist scan blind spot while the
1151 * task is exiting and may be removed from the tasklist. See
1152 * corresponding synchronize_srcu() for further details.
1154 void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
1156 current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
1160 * Contribute to protect against tasklist scan blind spot while the
1161 * task is exiting and may be removed from the tasklist. See
1162 * corresponding synchronize_srcu() for further details.
1164 void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
1166 struct task_struct *t = current;
1168 __srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
1172 * Contribute to protect against tasklist scan blind spot while the
1173 * task is exiting and may be removed from the tasklist. See
1174 * corresponding synchronize_srcu() for further details.
1176 void exit_tasks_rcu_finish(void)
1178 exit_tasks_rcu_stop();
1179 exit_tasks_rcu_finish_trace(current);
1182 #else /* #ifdef CONFIG_TASKS_RCU */
1183 void exit_tasks_rcu_start(void) { }
1184 void exit_tasks_rcu_stop(void) { }
1185 void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
1186 #endif /* #else #ifdef CONFIG_TASKS_RCU */
1188 #ifdef CONFIG_TASKS_RUDE_RCU
1190 ////////////////////////////////////////////////////////////////////////
1192 // "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
1193 // passing an empty function to schedule_on_each_cpu(). This approach
1194 // provides an asynchronous call_rcu_tasks_rude() API and batching of
1195 // concurrent calls to the synchronous synchronize_rcu_tasks_rude() API.
1196 // This invokes schedule_on_each_cpu() in order to send IPIs far and wide
1197 // and induces otherwise unnecessary context switches on all online CPUs,
1198 // whether idle or not.
1200 // Callback handling is provided by the rcu_tasks_kthread() function.
1202 // Ordering is provided by the scheduler's context-switch code.
1204 // Empty function to allow workqueues to force a context switch.
1205 static void rcu_tasks_be_rude(struct work_struct *work)
1209 // Wait for one rude RCU-tasks grace period.
1210 static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
1212 rtp->n_ipis += cpumask_weight(cpu_online_mask);
1213 schedule_on_each_cpu(rcu_tasks_be_rude);
1216 void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
1217 DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude,
1221 * call_rcu_tasks_rude() - Queue a callback rude task-based grace period
1222 * @rhp: structure to be used for queueing the RCU updates.
1223 * @func: actual callback function to be invoked after the grace period
1225 * The callback function will be invoked some time after a full grace
1226 * period elapses, in other words after all currently executing RCU
1227 * read-side critical sections have completed. call_rcu_tasks_rude()
1228 * assumes that the read-side critical sections end at context switch,
1229 * cond_resched_tasks_rcu_qs(), or transition to usermode execution (as
1230 * usermode execution is schedulable). As such, there are no read-side
1231 * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
1232 * this primitive is intended to determine that all tasks have passed
1233 * through a safe state, not so much for data-structure synchronization.
1235 * See the description of call_rcu() for more detailed information on
1236 * memory ordering guarantees.
1238 void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
1240 call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
1242 EXPORT_SYMBOL_GPL(call_rcu_tasks_rude);
1245 * synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
1247 * Control will return to the caller some time after a rude rcu-tasks
1248 * grace period has elapsed, in other words after all currently
1249 * executing rcu-tasks read-side critical sections have elapsed. These
1250 * read-side critical sections are delimited by calls to schedule(),
1251 * cond_resched_tasks_rcu_qs(), userspace execution (which is a schedulable
1252 * context), and (in theory, anyway) cond_resched().
1254 * This is a very specialized primitive, intended only for a few uses in
1255 * tracing and other situations requiring manipulation of function preambles
1256 * and profiling hooks. The synchronize_rcu_tasks_rude() function is not
1257 * (yet) intended for heavy use from multiple CPUs.
1259 * See the description of synchronize_rcu() for more detailed information
1260 * on memory ordering guarantees.
1262 void synchronize_rcu_tasks_rude(void)
1264 synchronize_rcu_tasks_generic(&rcu_tasks_rude);
1266 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);
1269 * rcu_barrier_tasks_rude - Wait for in-flight call_rcu_tasks_rude() callbacks.
1271 * Although the current implementation is guaranteed to wait, it is not
1272 * obligated to, for example, if there are no pending callbacks.
1274 void rcu_barrier_tasks_rude(void)
1276 rcu_barrier_tasks_generic(&rcu_tasks_rude);
1278 EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude);
1280 int rcu_tasks_rude_lazy_ms = -1;
1281 module_param(rcu_tasks_rude_lazy_ms, int, 0444);
1283 static int __init rcu_spawn_tasks_rude_kthread(void)
1285 cblist_init_generic(&rcu_tasks_rude);
1286 rcu_tasks_rude.gp_sleep = HZ / 10;
1287 if (rcu_tasks_rude_lazy_ms >= 0)
1288 rcu_tasks_rude.lazy_jiffies = msecs_to_jiffies(rcu_tasks_rude_lazy_ms);
1289 rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
1293 #if !defined(CONFIG_TINY_RCU)
1294 void show_rcu_tasks_rude_gp_kthread(void)
1296 show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, "");
1298 EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
1299 #endif // !defined(CONFIG_TINY_RCU)
1301 struct task_struct *get_rcu_tasks_rude_gp_kthread(void)
1303 return rcu_tasks_rude.kthread_ptr;
1305 EXPORT_SYMBOL_GPL(get_rcu_tasks_rude_gp_kthread);
1307 #endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
1309 ////////////////////////////////////////////////////////////////////////
1311 // Tracing variant of Tasks RCU. This variant is designed to be used
1312 // to protect tracing hooks, including those of BPF. This variant
1315 // 1. Has explicit read-side markers to allow finite grace periods
1316 // in the face of in-kernel loops for PREEMPT=n builds.
1318 // 2. Protects code in the idle loop, exception entry/exit, and
1319 // CPU-hotplug code paths, similar to the capabilities of SRCU.
1321 // 3. Avoids expensive read-side instructions, having overhead similar
1322 // to that of Preemptible RCU.
1324 // There are of course downsides. For example, the grace-period code
1325 // can send IPIs to CPUs, even when those CPUs are in the idle loop or
1326 // in nohz_full userspace. If needed, these downsides can be at least
1327 // partially remedied.
1329 // Perhaps most important, this variant of RCU does not affect the vanilla
1330 // flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace
1331 // readers can operate from idle, offline, and exception entry/exit in no
1332 // way allows rcu_preempt and rcu_sched readers to also do so.
1334 // The implementation uses rcu_tasks_wait_gp(), which relies on function
1335 // pointers in the rcu_tasks structure. The rcu_spawn_tasks_trace_kthread()
1336 // function sets these function pointers up so that rcu_tasks_wait_gp()
1337 // invokes these functions in this order:
1339 // rcu_tasks_trace_pregp_step():
1340 // Disables CPU hotplug, adds all currently executing tasks to the
1341 // holdout list, then checks the state of all tasks that blocked
1342 // or were preempted within their current RCU Tasks Trace read-side
1343 // critical section, adding them to the holdout list if appropriate.
1344 // Finally, this function re-enables CPU hotplug.
1345 // The ->pertask_func() pointer is NULL, so there is no per-task processing.
1346 // rcu_tasks_trace_postscan():
1347 // Invokes synchronize_rcu() to wait for late-stage exiting tasks
1348 // to finish exiting.
1349 // check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
1350 // Scans the holdout list, attempting to identify a quiescent state
1351 // for each task on the list. If there is a quiescent state, the
1352 // corresponding task is removed from the holdout list. Once this
1353 // list is empty, the grace period has completed.
1354 // rcu_tasks_trace_postgp():
1355 // Provides the needed full memory barrier and does debug checks.
1357 // The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
1359 // Pre-grace-period update-side code is ordered before the grace period
1360 // via the ->cbs_lock and barriers in rcu_tasks_kthread(). Pre-grace-period
1361 // read-side code is ordered before the grace period by atomic operations
1362 // on .b.need_qs flag of each task involved in this process, or by scheduler
1363 // context-switch ordering (for locked-down non-running readers).
1365 // The lockdep state must be outside of #ifdef to be useful.
1366 #ifdef CONFIG_DEBUG_LOCK_ALLOC
1367 static struct lock_class_key rcu_lock_trace_key;
1368 struct lockdep_map rcu_trace_lock_map =
1369 STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_trace", &rcu_lock_trace_key);
1370 EXPORT_SYMBOL_GPL(rcu_trace_lock_map);
1371 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
1373 #ifdef CONFIG_TASKS_TRACE_RCU
1375 // Record outstanding IPIs to each CPU. No point in sending two...
1376 static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);
1378 // The number of detections of task quiescent state relying on
1379 // heavyweight readers executing explicit memory barriers.
1380 static unsigned long n_heavy_reader_attempts;
1381 static unsigned long n_heavy_reader_updates;
1382 static unsigned long n_heavy_reader_ofl_updates;
1383 static unsigned long n_trc_holdouts;
1385 void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
1386 DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
1389 /* Load from ->trc_reader_special.b.need_qs with proper ordering. */
1390 static u8 rcu_ld_need_qs(struct task_struct *t)
1392 smp_mb(); // Enforce full grace-period ordering.
1393 return smp_load_acquire(&t->trc_reader_special.b.need_qs);
1396 /* Store to ->trc_reader_special.b.need_qs with proper ordering. */
1397 static void rcu_st_need_qs(struct task_struct *t, u8 v)
1399 smp_store_release(&t->trc_reader_special.b.need_qs, v);
1400 smp_mb(); // Enforce full grace-period ordering.
1404 * Do a cmpxchg() on ->trc_reader_special.b.need_qs, allowing for
1405 * the four-byte operand-size restriction of some platforms.
1406 * Returns the old value, which is often ignored.
1408 u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
1410 union rcu_special ret;
1411 union rcu_special trs_old = READ_ONCE(t->trc_reader_special);
1412 union rcu_special trs_new = trs_old;
1414 if (trs_old.b.need_qs != old)
1415 return trs_old.b.need_qs;
1416 trs_new.b.need_qs = new;
1417 ret.s = cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s);
1418 return ret.b.need_qs;
1420 EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);
1423 * If we are the last reader, signal the grace-period kthread.
1424 * Also remove from the per-CPU list of blocked tasks.
1426 void rcu_read_unlock_trace_special(struct task_struct *t)
1428 unsigned long flags;
1429 struct rcu_tasks_percpu *rtpcp;
1430 union rcu_special trs;
1432 // Open-coded full-word version of rcu_ld_need_qs().
1433 smp_mb(); // Enforce full grace-period ordering.
1434 trs = smp_load_acquire(&t->trc_reader_special);
1436 if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && t->trc_reader_special.b.need_mb)
1437 smp_mb(); // Pairs with update-side barriers.
1438 // Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
1439 if (trs.b.need_qs == (TRC_NEED_QS_CHECKED | TRC_NEED_QS)) {
1440 u8 result = rcu_trc_cmpxchg_need_qs(t, TRC_NEED_QS_CHECKED | TRC_NEED_QS,
1441 TRC_NEED_QS_CHECKED);
1443 WARN_ONCE(result != trs.b.need_qs, "%s: result = %d", __func__, result);
1445 if (trs.b.blocked) {
1446 rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, t->trc_blkd_cpu);
1447 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1448 list_del_init(&t->trc_blkd_node);
1449 WRITE_ONCE(t->trc_reader_special.b.blocked, false);
1450 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1452 WRITE_ONCE(t->trc_reader_nesting, 0);
1454 EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
1456 /* Add a newly blocked reader task to its CPU's list. */
1457 void rcu_tasks_trace_qs_blkd(struct task_struct *t)
1459 unsigned long flags;
1460 struct rcu_tasks_percpu *rtpcp;
1462 local_irq_save(flags);
1463 rtpcp = this_cpu_ptr(rcu_tasks_trace.rtpcpu);
1464 raw_spin_lock_rcu_node(rtpcp); // irqs already disabled
1465 t->trc_blkd_cpu = smp_processor_id();
1466 if (!rtpcp->rtp_blkd_tasks.next)
1467 INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
1468 list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
1469 WRITE_ONCE(t->trc_reader_special.b.blocked, true);
1470 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1472 EXPORT_SYMBOL_GPL(rcu_tasks_trace_qs_blkd);
1474 /* Add a task to the holdout list, if it is not already on the list. */
1475 static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
1477 if (list_empty(&t->trc_holdout_list)) {
1479 list_add(&t->trc_holdout_list, bhp);
1484 /* Remove a task from the holdout list, if it is in fact present. */
1485 static void trc_del_holdout(struct task_struct *t)
1487 if (!list_empty(&t->trc_holdout_list)) {
1488 list_del_init(&t->trc_holdout_list);
1494 /* IPI handler to check task state. */
1495 static void trc_read_check_handler(void *t_in)
1498 struct task_struct *t = current;
1499 struct task_struct *texp = t_in;
1501 // If the task is no longer running on this CPU, leave.
1502 if (unlikely(texp != t))
1503 goto reset_ipi; // Already on holdout list, so will check later.
1505 // If the task is not in a read-side critical section, and
1506 // if this is the last reader, awaken the grace-period kthread.
1507 nesting = READ_ONCE(t->trc_reader_nesting);
1508 if (likely(!nesting)) {
1509 rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
1512 // If we are racing with an rcu_read_unlock_trace(), try again later.
1513 if (unlikely(nesting < 0))
1516 // Get here if the task is in a read-side critical section.
1517 // Set its state so that it will update state for the grace-period
1518 // kthread upon exit from that critical section.
1519 rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);
1522 // Allow future IPIs to be sent on CPU and for task.
1523 // Also order this IPI handler against any later manipulations of
1524 // the intended task.
1525 smp_store_release(per_cpu_ptr(&trc_ipi_to_cpu, smp_processor_id()), false); // ^^^
1526 smp_store_release(&texp->trc_ipi_to_cpu, -1); // ^^^
1529 /* Callback function for scheduler to check locked-down task. */
1530 static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
1532 struct list_head *bhp = bhp_in;
1533 int cpu = task_cpu(t);
1535 bool ofl = cpu_is_offline(cpu);
1537 if (task_curr(t) && !ofl) {
1538 // If no chance of heavyweight readers, do it the hard way.
1539 if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
1542 // If heavyweight readers are enabled on the remote task,
1543 // we can inspect its state despite its currently running.
1544 // However, we cannot safely change its state.
1545 n_heavy_reader_attempts++;
1546 // Check for "running" idle tasks on offline CPUs.
1547 if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
1548 return -EINVAL; // No quiescent state, do it the hard way.
1549 n_heavy_reader_updates++;
1552 // The task is not running, so C-language access is safe.
1553 nesting = t->trc_reader_nesting;
1554 WARN_ON_ONCE(ofl && task_curr(t) && (t != idle_task(task_cpu(t))));
1555 if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && ofl)
1556 n_heavy_reader_ofl_updates++;
1559 // If not exiting a read-side critical section, mark as checked
1560 // so that the grace-period kthread will remove it from the
1563 rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
1564 return 0; // In QS, so done.
1567 return -EINVAL; // Reader transitioning, try again later.
1569 // The task is in a read-side critical section, so set up its
1570 // state so that it will update state upon exit from that critical
1572 if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED))
1573 trc_add_holdout(t, bhp);
1577 /* Attempt to extract the state for the specified task. */
1578 static void trc_wait_for_one_reader(struct task_struct *t,
1579 struct list_head *bhp)
1583 // If a previous IPI is still in flight, let it complete.
1584 if (smp_load_acquire(&t->trc_ipi_to_cpu) != -1) // Order IPI
1587 // The current task had better be in a quiescent state.
1589 rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
1590 WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
1594 // Attempt to nail down the task for inspection.
1596 if (!task_call_func(t, trc_inspect_reader, bhp)) {
1602 // If this task is not yet on the holdout list, then we are in
1603 // an RCU read-side critical section. Otherwise, the invocation of
1604 // trc_add_holdout() that added it to the list did the necessary
1605 // get_task_struct(). Either way, the task cannot be freed out
1606 // from under this code.
1608 // If currently running, send an IPI, either way, add to list.
1609 trc_add_holdout(t, bhp);
1611 time_after(jiffies + 1, rcu_tasks_trace.gp_start + rcu_task_ipi_delay)) {
1612 // The task is currently running, so try IPIing it.
1615 // If there is already an IPI outstanding, let it happen.
1616 if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0)
1619 per_cpu(trc_ipi_to_cpu, cpu) = true;
1620 t->trc_ipi_to_cpu = cpu;
1621 rcu_tasks_trace.n_ipis++;
1622 if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) {
1623 // Just in case there is some other reason for
1624 // failure than the target CPU being offline.
1625 WARN_ONCE(1, "%s(): smp_call_function_single() failed for CPU: %d\n",
1627 rcu_tasks_trace.n_ipis_fails++;
1628 per_cpu(trc_ipi_to_cpu, cpu) = false;
1629 t->trc_ipi_to_cpu = -1;
1635 * Initialize for first-round processing for the specified task.
1636 * Return false if task is NULL or already taken care of, true otherwise.
1638 static bool rcu_tasks_trace_pertask_prep(struct task_struct *t, bool notself)
1640 // During early boot when there is only the one boot CPU, there
1641 // is no idle task for the other CPUs. Also, the grace-period
1642 // kthread is always in a quiescent state. In addition, just return
1643 // if this task is already on the list.
1644 if (unlikely(t == NULL) || (t == current && notself) || !list_empty(&t->trc_holdout_list))
1647 rcu_st_need_qs(t, 0);
1648 t->trc_ipi_to_cpu = -1;
1652 /* Do first-round processing for the specified task. */
1653 static void rcu_tasks_trace_pertask(struct task_struct *t, struct list_head *hop)
1655 if (rcu_tasks_trace_pertask_prep(t, true))
1656 trc_wait_for_one_reader(t, hop);
1659 /* Initialize for a new RCU-tasks-trace grace period. */
1660 static void rcu_tasks_trace_pregp_step(struct list_head *hop)
1662 LIST_HEAD(blkd_tasks);
1664 unsigned long flags;
1665 struct rcu_tasks_percpu *rtpcp;
1666 struct task_struct *t;
1668 // There shouldn't be any old IPIs, but...
1669 for_each_possible_cpu(cpu)
1670 WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
1672 // Disable CPU hotplug across the CPU scan for the benefit of
1673 // any IPIs that might be needed. This also waits for all readers
1674 // in CPU-hotplug code paths.
1677 // These rcu_tasks_trace_pertask_prep() calls are serialized to
1678 // allow safe access to the hop list.
1679 for_each_online_cpu(cpu) {
1681 t = cpu_curr_snapshot(cpu);
1682 if (rcu_tasks_trace_pertask_prep(t, true))
1683 trc_add_holdout(t, hop);
1685 cond_resched_tasks_rcu_qs();
1688 // Only after all running tasks have been accounted for is it
1689 // safe to take care of the tasks that have blocked within their
1690 // current RCU tasks trace read-side critical section.
1691 for_each_possible_cpu(cpu) {
1692 rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, cpu);
1693 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1694 list_splice_init(&rtpcp->rtp_blkd_tasks, &blkd_tasks);
1695 while (!list_empty(&blkd_tasks)) {
1697 t = list_first_entry(&blkd_tasks, struct task_struct, trc_blkd_node);
1698 list_del_init(&t->trc_blkd_node);
1699 list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
1700 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1701 rcu_tasks_trace_pertask(t, hop);
1703 raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
1705 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
1706 cond_resched_tasks_rcu_qs();
1709 // Re-enable CPU hotplug now that the holdout list is populated.
1714 * Do intermediate processing between task and holdout scans.
1716 static void rcu_tasks_trace_postscan(struct list_head *hop)
1718 // Wait for late-stage exiting tasks to finish exiting.
1719 // These might have passed the call to exit_tasks_rcu_finish().
1721 // If you remove the following line, update rcu_trace_implies_rcu_gp()!!!
1723 // Any tasks that exit after this point will set
1724 // TRC_NEED_QS_CHECKED in ->trc_reader_special.b.need_qs.
1727 /* Communicate task state back to the RCU tasks trace stall warning request. */
1728 struct trc_stall_chk_rdr {
1734 static int trc_check_slow_task(struct task_struct *t, void *arg)
1736 struct trc_stall_chk_rdr *trc_rdrp = arg;
1738 if (task_curr(t) && cpu_online(task_cpu(t)))
1739 return false; // It is running, so decline to inspect it.
1740 trc_rdrp->nesting = READ_ONCE(t->trc_reader_nesting);
1741 trc_rdrp->ipi_to_cpu = READ_ONCE(t->trc_ipi_to_cpu);
1742 trc_rdrp->needqs = rcu_ld_need_qs(t);
1746 /* Show the state of a task stalling the current RCU tasks trace GP. */
1747 static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
1750 struct trc_stall_chk_rdr trc_rdr;
1751 bool is_idle_tsk = is_idle_task(t);
1754 pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n");
1755 *firstreport = false;
1758 if (!task_call_func(t, trc_check_slow_task, &trc_rdr))
1759 pr_alert("P%d: %c%c\n",
1761 ".I"[t->trc_ipi_to_cpu >= 0],
1764 pr_alert("P%d: %c%c%c%c nesting: %d%c%c cpu: %d%s\n",
1766 ".I"[trc_rdr.ipi_to_cpu >= 0],
1768 ".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
1769 ".B"[!!data_race(t->trc_reader_special.b.blocked)],
1771 " !CN"[trc_rdr.needqs & 0x3],
1772 " ?"[trc_rdr.needqs > 0x3],
1773 cpu, cpu_online(cpu) ? "" : "(offline)");
1777 /* List stalled IPIs for RCU tasks trace. */
1778 static void show_stalled_ipi_trace(void)
1782 for_each_possible_cpu(cpu)
1783 if (per_cpu(trc_ipi_to_cpu, cpu))
1784 pr_alert("\tIPI outstanding to CPU %d\n", cpu);
1787 /* Do one scan of the holdout list. */
1788 static void check_all_holdout_tasks_trace(struct list_head *hop,
1789 bool needreport, bool *firstreport)
1791 struct task_struct *g, *t;
1793 // Disable CPU hotplug across the holdout list scan for IPIs.
1796 list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
1797 // If safe and needed, try to check the current task.
1798 if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
1799 !(rcu_ld_need_qs(t) & TRC_NEED_QS_CHECKED))
1800 trc_wait_for_one_reader(t, hop);
1802 // If check succeeded, remove this task from the list.
1803 if (smp_load_acquire(&t->trc_ipi_to_cpu) == -1 &&
1804 rcu_ld_need_qs(t) == TRC_NEED_QS_CHECKED)
1806 else if (needreport)
1807 show_stalled_task_trace(t, firstreport);
1808 cond_resched_tasks_rcu_qs();
1811 // Re-enable CPU hotplug now that the holdout list scan has completed.
1816 pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
1817 show_stalled_ipi_trace();
1821 static void rcu_tasks_trace_empty_fn(void *unused)
1825 /* Wait for grace period to complete and provide ordering. */
1826 static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
1830 // Wait for any lingering IPI handlers to complete. Note that
1831 // if a CPU has gone offline or transitioned to userspace in the
1832 // meantime, all IPI handlers should have been drained beforehand.
1833 // Yes, this assumes that CPUs process IPIs in order. If that ever
1834 // changes, there will need to be a recheck and/or timed wait.
1835 for_each_online_cpu(cpu)
1836 if (WARN_ON_ONCE(smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu))))
1837 smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1);
1839 smp_mb(); // Caller's code must be ordered after wakeup.
1840 // Pairs with pretty much every ordering primitive.
1843 /* Report any needed quiescent state for this exiting task. */
1844 static void exit_tasks_rcu_finish_trace(struct task_struct *t)
1846 union rcu_special trs = READ_ONCE(t->trc_reader_special);
1848 rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
1849 WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
1850 if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS || trs.b.blocked))
1851 rcu_read_unlock_trace_special(t);
1853 WRITE_ONCE(t->trc_reader_nesting, 0);
1857 * call_rcu_tasks_trace() - Queue a callback trace task-based grace period
1858 * @rhp: structure to be used for queueing the RCU updates.
1859 * @func: actual callback function to be invoked after the grace period
1861 * The callback function will be invoked some time after a trace rcu-tasks
1862 * grace period elapses, in other words after all currently executing
1863 * trace rcu-tasks read-side critical sections have completed. These
1864 * read-side critical sections are delimited by calls to rcu_read_lock_trace()
1865 * and rcu_read_unlock_trace().
1867 * See the description of call_rcu() for more detailed information on
1868 * memory ordering guarantees.
1870 void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func)
1872 call_rcu_tasks_generic(rhp, func, &rcu_tasks_trace);
1874 EXPORT_SYMBOL_GPL(call_rcu_tasks_trace);
1877 * synchronize_rcu_tasks_trace - wait for a trace rcu-tasks grace period
1879 * Control will return to the caller some time after a trace rcu-tasks
1880 * grace period has elapsed, in other words after all currently executing
1881 * trace rcu-tasks read-side critical sections have elapsed. These read-side
1882 * critical sections are delimited by calls to rcu_read_lock_trace()
1883 * and rcu_read_unlock_trace().
1885 * This is a very specialized primitive, intended only for a few uses in
1886 * tracing and other situations requiring manipulation of function preambles
1887 * and profiling hooks. The synchronize_rcu_tasks_trace() function is not
1888 * (yet) intended for heavy use from multiple CPUs.
1890 * See the description of synchronize_rcu() for more detailed information
1891 * on memory ordering guarantees.
1893 void synchronize_rcu_tasks_trace(void)
1895 RCU_LOCKDEP_WARN(lock_is_held(&rcu_trace_lock_map), "Illegal synchronize_rcu_tasks_trace() in RCU Tasks Trace read-side critical section");
1896 synchronize_rcu_tasks_generic(&rcu_tasks_trace);
1898 EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_trace);
1901 * rcu_barrier_tasks_trace - Wait for in-flight call_rcu_tasks_trace() callbacks.
1903 * Although the current implementation is guaranteed to wait, it is not
1904 * obligated to, for example, if there are no pending callbacks.
1906 void rcu_barrier_tasks_trace(void)
1908 rcu_barrier_tasks_generic(&rcu_tasks_trace);
1910 EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace);
1912 int rcu_tasks_trace_lazy_ms = -1;
1913 module_param(rcu_tasks_trace_lazy_ms, int, 0444);
1915 static int __init rcu_spawn_tasks_trace_kthread(void)
1917 cblist_init_generic(&rcu_tasks_trace);
1918 if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
1919 rcu_tasks_trace.gp_sleep = HZ / 10;
1920 rcu_tasks_trace.init_fract = HZ / 10;
1922 rcu_tasks_trace.gp_sleep = HZ / 200;
1923 if (rcu_tasks_trace.gp_sleep <= 0)
1924 rcu_tasks_trace.gp_sleep = 1;
1925 rcu_tasks_trace.init_fract = HZ / 200;
1926 if (rcu_tasks_trace.init_fract <= 0)
1927 rcu_tasks_trace.init_fract = 1;
1929 if (rcu_tasks_trace_lazy_ms >= 0)
1930 rcu_tasks_trace.lazy_jiffies = msecs_to_jiffies(rcu_tasks_trace_lazy_ms);
1931 rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step;
1932 rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan;
1933 rcu_tasks_trace.holdouts_func = check_all_holdout_tasks_trace;
1934 rcu_tasks_trace.postgp_func = rcu_tasks_trace_postgp;
1935 rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
1939 #if !defined(CONFIG_TINY_RCU)
1940 void show_rcu_tasks_trace_gp_kthread(void)
1944 sprintf(buf, "N%lu h:%lu/%lu/%lu",
1945 data_race(n_trc_holdouts),
1946 data_race(n_heavy_reader_ofl_updates),
1947 data_race(n_heavy_reader_updates),
1948 data_race(n_heavy_reader_attempts));
1949 show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);
1951 EXPORT_SYMBOL_GPL(show_rcu_tasks_trace_gp_kthread);
1952 #endif // !defined(CONFIG_TINY_RCU)
1954 struct task_struct *get_rcu_tasks_trace_gp_kthread(void)
1956 return rcu_tasks_trace.kthread_ptr;
1958 EXPORT_SYMBOL_GPL(get_rcu_tasks_trace_gp_kthread);
1960 #else /* #ifdef CONFIG_TASKS_TRACE_RCU */
1961 static void exit_tasks_rcu_finish_trace(struct task_struct *t) { }
1962 #endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */
1964 #ifndef CONFIG_TINY_RCU
1965 void show_rcu_tasks_gp_kthreads(void)
1967 show_rcu_tasks_classic_gp_kthread();
1968 show_rcu_tasks_rude_gp_kthread();
1969 show_rcu_tasks_trace_gp_kthread();
1971 #endif /* #ifndef CONFIG_TINY_RCU */
1973 #ifdef CONFIG_PROVE_RCU
1974 struct rcu_tasks_test_desc {
1978 unsigned long runstart;
1981 static struct rcu_tasks_test_desc tests[] = {
1983 .name = "call_rcu_tasks()",
1984 /* If not defined, the test is skipped. */
1985 .notrun = IS_ENABLED(CONFIG_TASKS_RCU),
1988 .name = "call_rcu_tasks_rude()",
1989 /* If not defined, the test is skipped. */
1990 .notrun = IS_ENABLED(CONFIG_TASKS_RUDE_RCU),
1993 .name = "call_rcu_tasks_trace()",
1994 /* If not defined, the test is skipped. */
1995 .notrun = IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
1999 static void test_rcu_tasks_callback(struct rcu_head *rhp)
2001 struct rcu_tasks_test_desc *rttd =
2002 container_of(rhp, struct rcu_tasks_test_desc, rh);
2004 pr_info("Callback from %s invoked.\n", rttd->name);
2006 rttd->notrun = false;
2009 static void rcu_tasks_initiate_self_tests(void)
2011 #ifdef CONFIG_TASKS_RCU
2012 pr_info("Running RCU Tasks wait API self tests\n");
2013 tests[0].runstart = jiffies;
2014 synchronize_rcu_tasks();
2015 call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
2018 #ifdef CONFIG_TASKS_RUDE_RCU
2019 pr_info("Running RCU Tasks Rude wait API self tests\n");
2020 tests[1].runstart = jiffies;
2021 synchronize_rcu_tasks_rude();
2022 call_rcu_tasks_rude(&tests[1].rh, test_rcu_tasks_callback);
2025 #ifdef CONFIG_TASKS_TRACE_RCU
2026 pr_info("Running RCU Tasks Trace wait API self tests\n");
2027 tests[2].runstart = jiffies;
2028 synchronize_rcu_tasks_trace();
2029 call_rcu_tasks_trace(&tests[2].rh, test_rcu_tasks_callback);
2034 * Return: 0 - test passed
2035 * 1 - test failed, but have not timed out yet
2036 * -1 - test failed and timed out
2038 static int rcu_tasks_verify_self_tests(void)
2042 unsigned long bst = rcu_task_stall_timeout;
2044 if (bst <= 0 || bst > RCU_TASK_BOOT_STALL_TIMEOUT)
2045 bst = RCU_TASK_BOOT_STALL_TIMEOUT;
2046 for (i = 0; i < ARRAY_SIZE(tests); i++) {
2047 while (tests[i].notrun) { // still hanging.
2048 if (time_after(jiffies, tests[i].runstart + bst)) {
2049 pr_err("%s has failed boot-time tests.\n", tests[i].name);
2063 * Repeat the rcu_tasks_verify_self_tests() call once every second until the
2064 * test passes or has timed out.
2066 static struct delayed_work rcu_tasks_verify_work;
2067 static void rcu_tasks_verify_work_fn(struct work_struct *work __maybe_unused)
2069 int ret = rcu_tasks_verify_self_tests();
2074 /* Test fails but not timed out yet, reschedule another check */
2075 schedule_delayed_work(&rcu_tasks_verify_work, HZ);
2078 static int rcu_tasks_verify_schedule_work(void)
2080 INIT_DELAYED_WORK(&rcu_tasks_verify_work, rcu_tasks_verify_work_fn);
2081 rcu_tasks_verify_work_fn(NULL);
2084 late_initcall(rcu_tasks_verify_schedule_work);
2085 #else /* #ifdef CONFIG_PROVE_RCU */
2086 static void rcu_tasks_initiate_self_tests(void) { }
2087 #endif /* #else #ifdef CONFIG_PROVE_RCU */
2089 void __init rcu_init_tasks_generic(void)
2091 #ifdef CONFIG_TASKS_RCU
2092 rcu_spawn_tasks_kthread();
2095 #ifdef CONFIG_TASKS_RUDE_RCU
2096 rcu_spawn_tasks_rude_kthread();
2099 #ifdef CONFIG_TASKS_TRACE_RCU
2100 rcu_spawn_tasks_trace_kthread();
2103 // Run the self-tests.
2104 rcu_tasks_initiate_self_tests();
2107 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
2108 static inline void rcu_tasks_bootup_oddness(void) {}
2109 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */