Commit | Line | Data |
---|---|---|
9390ef0c | 1 | #include <linux/atomic.h> |
a1fd3e24 ON |
2 | #include <linux/rwsem.h> |
3 | #include <linux/percpu.h> | |
4 | #include <linux/wait.h> | |
8ebe3473 | 5 | #include <linux/lockdep.h> |
a1fd3e24 ON |
6 | #include <linux/percpu-rwsem.h> |
7 | #include <linux/rcupdate.h> | |
8 | #include <linux/sched.h> | |
9 | #include <linux/errno.h> | |
10 | ||
8ebe3473 ON |
11 | int __percpu_init_rwsem(struct percpu_rw_semaphore *brw, |
12 | const char *name, struct lock_class_key *rwsem_key) | |
a1fd3e24 ON |
13 | { |
14 | brw->fast_read_ctr = alloc_percpu(int); | |
15 | if (unlikely(!brw->fast_read_ctr)) | |
16 | return -ENOMEM; | |
17 | ||
8ebe3473 ON |
18 | /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ |
19 | __init_rwsem(&brw->rw_sem, name, rwsem_key); | |
001dac62 | 20 | rcu_sync_init(&brw->rss, RCU_SCHED_SYNC); |
a1fd3e24 ON |
21 | atomic_set(&brw->slow_read_ctr, 0); |
22 | init_waitqueue_head(&brw->write_waitq); | |
23 | return 0; | |
24 | } | |
302707fd | 25 | EXPORT_SYMBOL_GPL(__percpu_init_rwsem); |
a1fd3e24 ON |
26 | |
27 | void percpu_free_rwsem(struct percpu_rw_semaphore *brw) | |
28 | { | |
95b19f68 ON |
29 | /* |
30 | * XXX: temporary kludge. The error path in alloc_super() | |
31 | * assumes that percpu_free_rwsem() is safe after kzalloc(). | |
32 | */ | |
33 | if (!brw->fast_read_ctr) | |
34 | return; | |
35 | ||
001dac62 | 36 | rcu_sync_dtor(&brw->rss); |
a1fd3e24 ON |
37 | free_percpu(brw->fast_read_ctr); |
38 | brw->fast_read_ctr = NULL; /* catch use after free bugs */ | |
39 | } | |
40 | ||
41 | /* | |
f324a763 ON |
42 | * This is the fast-path for down_read/up_read. If it succeeds we rely |
43 | * on the barriers provided by rcu_sync_enter/exit; see the comments in | |
44 | * percpu_down_write() and percpu_up_write(). | |
a1fd3e24 ON |
45 | * |
46 | * If this helper fails the callers rely on the normal rw_semaphore and | |
47 | * atomic_dec_and_test(), so in this case we have the necessary barriers. | |
a1fd3e24 ON |
48 | */ |
49 | static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) | |
50 | { | |
001dac62 | 51 | bool success; |
a1fd3e24 ON |
52 | |
53 | preempt_disable(); | |
001dac62 ON |
54 | success = rcu_sync_is_idle(&brw->rss); |
55 | if (likely(success)) | |
a1fd3e24 | 56 | __this_cpu_add(*brw->fast_read_ctr, val); |
a1fd3e24 ON |
57 | preempt_enable(); |
58 | ||
59 | return success; | |
60 | } | |
61 | ||
62 | /* | |
63 | * Like the normal down_read() this is not recursive, the writer can | |
64 | * come after the first percpu_down_read() and create the deadlock. | |
8ebe3473 ON |
65 | * |
66 | * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep, | |
67 | * percpu_up_read() does rwsem_release(). This pairs with the usage | |
68 | * of ->rw_sem in percpu_down/up_write(). | |
a1fd3e24 ON |
69 | */ |
70 | void percpu_down_read(struct percpu_rw_semaphore *brw) | |
71 | { | |
8ebe3473 | 72 | might_sleep(); |
cc5f730b ON |
73 | rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_); |
74 | ||
75 | if (likely(update_fast_ctr(brw, +1))) | |
a1fd3e24 ON |
76 | return; |
77 | ||
cc5f730b ON |
78 | /* Avoid rwsem_acquire_read() and rwsem_release() */ |
79 | __down_read(&brw->rw_sem); | |
a1fd3e24 | 80 | atomic_inc(&brw->slow_read_ctr); |
8ebe3473 | 81 | __up_read(&brw->rw_sem); |
a1fd3e24 | 82 | } |
302707fd | 83 | EXPORT_SYMBOL_GPL(percpu_down_read); |
a1fd3e24 | 84 | |
9287f692 ON |
85 | int percpu_down_read_trylock(struct percpu_rw_semaphore *brw) |
86 | { | |
87 | if (unlikely(!update_fast_ctr(brw, +1))) { | |
88 | if (!__down_read_trylock(&brw->rw_sem)) | |
89 | return 0; | |
90 | atomic_inc(&brw->slow_read_ctr); | |
91 | __up_read(&brw->rw_sem); | |
92 | } | |
93 | ||
94 | rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_); | |
95 | return 1; | |
96 | } | |
97 | ||
a1fd3e24 ON |
98 | void percpu_up_read(struct percpu_rw_semaphore *brw) |
99 | { | |
8ebe3473 ON |
100 | rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_); |
101 | ||
a1fd3e24 ON |
102 | if (likely(update_fast_ctr(brw, -1))) |
103 | return; | |
104 | ||
105 | /* false-positive is possible but harmless */ | |
106 | if (atomic_dec_and_test(&brw->slow_read_ctr)) | |
107 | wake_up_all(&brw->write_waitq); | |
108 | } | |
302707fd | 109 | EXPORT_SYMBOL_GPL(percpu_up_read); |
a1fd3e24 ON |
110 | |
111 | static int clear_fast_ctr(struct percpu_rw_semaphore *brw) | |
112 | { | |
113 | unsigned int sum = 0; | |
114 | int cpu; | |
115 | ||
116 | for_each_possible_cpu(cpu) { | |
117 | sum += per_cpu(*brw->fast_read_ctr, cpu); | |
118 | per_cpu(*brw->fast_read_ctr, cpu) = 0; | |
119 | } | |
120 | ||
121 | return sum; | |
122 | } | |
123 | ||
a1fd3e24 ON |
124 | void percpu_down_write(struct percpu_rw_semaphore *brw) |
125 | { | |
a1fd3e24 | 126 | /* |
f324a763 ON |
127 | * Make rcu_sync_is_idle() == F and thus disable the fast-path in |
128 | * percpu_down_read() and percpu_up_read(), and wait for gp pass. | |
a1fd3e24 | 129 | * |
f324a763 ON |
130 | * The latter synchronises us with the preceding readers which used |
131 | * the fast-past, so we can not miss the result of __this_cpu_add() | |
132 | * or anything else inside their criticial sections. | |
a1fd3e24 | 133 | */ |
001dac62 | 134 | rcu_sync_enter(&brw->rss); |
a1fd3e24 | 135 | |
9390ef0c ON |
136 | /* exclude other writers, and block the new readers completely */ |
137 | down_write(&brw->rw_sem); | |
138 | ||
a1fd3e24 ON |
139 | /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */ |
140 | atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); | |
141 | ||
a1fd3e24 ON |
142 | /* wait for all readers to complete their percpu_up_read() */ |
143 | wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr)); | |
144 | } | |
302707fd | 145 | EXPORT_SYMBOL_GPL(percpu_down_write); |
a1fd3e24 ON |
146 | |
147 | void percpu_up_write(struct percpu_rw_semaphore *brw) | |
148 | { | |
9390ef0c | 149 | /* release the lock, but the readers can't use the fast-path */ |
a1fd3e24 | 150 | up_write(&brw->rw_sem); |
a1fd3e24 | 151 | /* |
f324a763 ON |
152 | * Enable the fast-path in percpu_down_read() and percpu_up_read() |
153 | * but only after another gp pass; this adds the necessary barrier | |
154 | * to ensure the reader can't miss the changes done by us. | |
a1fd3e24 | 155 | */ |
001dac62 | 156 | rcu_sync_exit(&brw->rss); |
a1fd3e24 | 157 | } |
302707fd | 158 | EXPORT_SYMBOL_GPL(percpu_up_write); |