Commit | Line | Data |
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1da177e4 LT |
1 | /* rwsem.c: R/W semaphores: contention handling functions |
2 | * | |
3 | * Written by David Howells (dhowells@redhat.com). | |
4 | * Derived from arch/i386/kernel/semaphore.c | |
ce6711f3 AS |
5 | * |
6 | * Writer lock-stealing by Alex Shi <alex.shi@intel.com> | |
1da177e4 LT |
7 | */ |
8 | #include <linux/rwsem.h> | |
9 | #include <linux/sched.h> | |
10 | #include <linux/init.h> | |
8bc3bcc9 | 11 | #include <linux/export.h> |
1da177e4 | 12 | |
4ea2176d IM |
13 | /* |
14 | * Initialize an rwsem: | |
15 | */ | |
16 | void __init_rwsem(struct rw_semaphore *sem, const char *name, | |
17 | struct lock_class_key *key) | |
18 | { | |
19 | #ifdef CONFIG_DEBUG_LOCK_ALLOC | |
20 | /* | |
21 | * Make sure we are not reinitializing a held semaphore: | |
22 | */ | |
23 | debug_check_no_locks_freed((void *)sem, sizeof(*sem)); | |
4dfbb9d8 | 24 | lockdep_init_map(&sem->dep_map, name, key, 0); |
4ea2176d IM |
25 | #endif |
26 | sem->count = RWSEM_UNLOCKED_VALUE; | |
ddb6c9b5 | 27 | raw_spin_lock_init(&sem->wait_lock); |
4ea2176d IM |
28 | INIT_LIST_HEAD(&sem->wait_list); |
29 | } | |
30 | ||
31 | EXPORT_SYMBOL(__init_rwsem); | |
32 | ||
e2d57f78 ML |
33 | enum rwsem_waiter_type { |
34 | RWSEM_WAITING_FOR_WRITE, | |
35 | RWSEM_WAITING_FOR_READ | |
36 | }; | |
37 | ||
1da177e4 LT |
38 | struct rwsem_waiter { |
39 | struct list_head list; | |
40 | struct task_struct *task; | |
e2d57f78 | 41 | enum rwsem_waiter_type type; |
1da177e4 LT |
42 | }; |
43 | ||
70bdc6e0 ML |
44 | /* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and |
45 | * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held | |
46 | * since the rwsem value was observed. | |
47 | */ | |
48 | #define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */ | |
49 | #define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */ | |
50 | #define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */ | |
51 | ||
1da177e4 LT |
52 | /* |
53 | * handle the lock release when processes blocked on it that can now run | |
54 | * - if we come here from up_xxxx(), then: | |
55 | * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) | |
56 | * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) | |
345af7bf | 57 | * - there must be someone on the queue |
1da177e4 LT |
58 | * - the spinlock must be held by the caller |
59 | * - woken process blocks are discarded from the list after having task zeroed | |
60 | * - writers are only woken if downgrading is false | |
61 | */ | |
70bdc6e0 ML |
62 | static struct rw_semaphore * |
63 | __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) | |
1da177e4 LT |
64 | { |
65 | struct rwsem_waiter *waiter; | |
66 | struct task_struct *tsk; | |
67 | struct list_head *next; | |
ce6711f3 | 68 | signed long woken, loop, adjustment; |
1da177e4 | 69 | |
345af7bf | 70 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); |
8cf5322c ML |
71 | if (waiter->type == RWSEM_WAITING_FOR_WRITE) { |
72 | if (wake_type != RWSEM_WAKE_READ_OWNED) | |
73 | /* Wake writer at the front of the queue, but do not | |
74 | * grant it the lock yet as we want other writers | |
75 | * to be able to steal it. Readers, on the other hand, | |
76 | * will block as they will notice the queued writer. | |
77 | */ | |
78 | wake_up_process(waiter->task); | |
345af7bf | 79 | goto out; |
8cf5322c | 80 | } |
1da177e4 | 81 | |
70bdc6e0 ML |
82 | /* If we come here from up_xxxx(), another thread might have reached |
83 | * rwsem_down_failed_common() before we acquired the spinlock and | |
84 | * woken up a waiter, making it now active. We prefer to check for | |
85 | * this first in order to not spend too much time with the spinlock | |
86 | * held if we're not going to be able to wake up readers in the end. | |
87 | * | |
88 | * Note that we do not need to update the rwsem count: any writer | |
89 | * trying to acquire rwsem will run rwsem_down_write_failed() due | |
90 | * to the waiting threads and block trying to acquire the spinlock. | |
91 | * | |
92 | * We use a dummy atomic update in order to acquire the cache line | |
93 | * exclusively since we expect to succeed and run the final rwsem | |
94 | * count adjustment pretty soon. | |
95 | */ | |
96 | if (wake_type == RWSEM_WAKE_ANY && | |
424acaae ML |
97 | rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS) |
98 | /* Someone grabbed the sem for write already */ | |
70bdc6e0 | 99 | goto out; |
1da177e4 | 100 | |
345af7bf ML |
101 | /* Grant an infinite number of read locks to the readers at the front |
102 | * of the queue. Note we increment the 'active part' of the count by | |
103 | * the number of readers before waking any processes up. | |
1da177e4 | 104 | */ |
1da177e4 LT |
105 | woken = 0; |
106 | do { | |
107 | woken++; | |
108 | ||
109 | if (waiter->list.next == &sem->wait_list) | |
110 | break; | |
111 | ||
112 | waiter = list_entry(waiter->list.next, | |
113 | struct rwsem_waiter, list); | |
114 | ||
e2d57f78 | 115 | } while (waiter->type != RWSEM_WAITING_FOR_WRITE); |
1da177e4 | 116 | |
fd41b334 | 117 | adjustment = woken * RWSEM_ACTIVE_READ_BIAS; |
e2d57f78 | 118 | if (waiter->type != RWSEM_WAITING_FOR_WRITE) |
fd41b334 ML |
119 | /* hit end of list above */ |
120 | adjustment -= RWSEM_WAITING_BIAS; | |
1da177e4 | 121 | |
fd41b334 | 122 | rwsem_atomic_add(adjustment, sem); |
1da177e4 LT |
123 | |
124 | next = sem->wait_list.next; | |
8cf5322c ML |
125 | loop = woken; |
126 | do { | |
1da177e4 LT |
127 | waiter = list_entry(next, struct rwsem_waiter, list); |
128 | next = waiter->list.next; | |
129 | tsk = waiter->task; | |
d59dd462 | 130 | smp_mb(); |
1da177e4 LT |
131 | waiter->task = NULL; |
132 | wake_up_process(tsk); | |
133 | put_task_struct(tsk); | |
8cf5322c | 134 | } while (--loop); |
1da177e4 LT |
135 | |
136 | sem->wait_list.next = next; | |
137 | next->prev = &sem->wait_list; | |
138 | ||
139 | out: | |
1da177e4 | 140 | return sem; |
ce6711f3 AS |
141 | } |
142 | ||
1da177e4 | 143 | /* |
1e78277c | 144 | * wait for the read lock to be granted |
1da177e4 | 145 | */ |
1e78277c | 146 | struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) |
1da177e4 | 147 | { |
1e78277c | 148 | signed long adjustment = -RWSEM_ACTIVE_READ_BIAS; |
a8618a0e | 149 | struct rwsem_waiter waiter; |
1da177e4 LT |
150 | struct task_struct *tsk = current; |
151 | signed long count; | |
152 | ||
1da177e4 | 153 | /* set up my own style of waitqueue */ |
a8618a0e | 154 | waiter.task = tsk; |
da16922c | 155 | waiter.type = RWSEM_WAITING_FOR_READ; |
1da177e4 LT |
156 | get_task_struct(tsk); |
157 | ||
f7dd1cee | 158 | raw_spin_lock_irq(&sem->wait_lock); |
fd41b334 ML |
159 | if (list_empty(&sem->wait_list)) |
160 | adjustment += RWSEM_WAITING_BIAS; | |
a8618a0e | 161 | list_add_tail(&waiter.list, &sem->wait_list); |
1da177e4 | 162 | |
70bdc6e0 | 163 | /* we're now waiting on the lock, but no longer actively locking */ |
1da177e4 LT |
164 | count = rwsem_atomic_update(adjustment, sem); |
165 | ||
da16922c | 166 | /* If there are no active locks, wake the front queued process(es). */ |
424acaae | 167 | if (count == RWSEM_WAITING_BIAS) |
70bdc6e0 | 168 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); |
1da177e4 | 169 | |
ddb6c9b5 | 170 | raw_spin_unlock_irq(&sem->wait_lock); |
1da177e4 LT |
171 | |
172 | /* wait to be given the lock */ | |
f7dd1cee ML |
173 | while (true) { |
174 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
a8618a0e | 175 | if (!waiter.task) |
1da177e4 LT |
176 | break; |
177 | schedule(); | |
1da177e4 LT |
178 | } |
179 | ||
180 | tsk->state = TASK_RUNNING; | |
181 | ||
182 | return sem; | |
183 | } | |
184 | ||
1da177e4 | 185 | /* |
023fe4f7 | 186 | * wait until we successfully acquire the write lock |
1da177e4 | 187 | */ |
d1233754 | 188 | struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) |
1da177e4 | 189 | { |
1e78277c ML |
190 | signed long adjustment = -RWSEM_ACTIVE_WRITE_BIAS; |
191 | struct rwsem_waiter waiter; | |
192 | struct task_struct *tsk = current; | |
193 | signed long count; | |
194 | ||
195 | /* set up my own style of waitqueue */ | |
196 | waiter.task = tsk; | |
023fe4f7 | 197 | waiter.type = RWSEM_WAITING_FOR_WRITE; |
1e78277c ML |
198 | |
199 | raw_spin_lock_irq(&sem->wait_lock); | |
200 | if (list_empty(&sem->wait_list)) | |
201 | adjustment += RWSEM_WAITING_BIAS; | |
202 | list_add_tail(&waiter.list, &sem->wait_list); | |
203 | ||
204 | /* we're now waiting on the lock, but no longer actively locking */ | |
205 | count = rwsem_atomic_update(adjustment, sem); | |
206 | ||
ed00f643 ML |
207 | /* If there were already threads queued before us and there are no |
208 | * active writers, the lock must be read owned; so we try to wake | |
209 | * any read locks that were queued ahead of us. */ | |
210 | if (count > RWSEM_WAITING_BIAS && | |
211 | adjustment == -RWSEM_ACTIVE_WRITE_BIAS) | |
1e78277c ML |
212 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
213 | ||
023fe4f7 | 214 | /* wait until we successfully acquire the lock */ |
a7d2c573 | 215 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
1e78277c | 216 | while (true) { |
9b0fc9c0 ML |
217 | if (!(count & RWSEM_ACTIVE_MASK)) { |
218 | /* Try acquiring the write lock. */ | |
219 | count = RWSEM_ACTIVE_WRITE_BIAS; | |
220 | if (!list_is_singular(&sem->wait_list)) | |
221 | count += RWSEM_WAITING_BIAS; | |
222 | if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) == | |
5ede972d | 223 | RWSEM_WAITING_BIAS) |
9b0fc9c0 ML |
224 | break; |
225 | } | |
1e78277c | 226 | |
1e78277c | 227 | raw_spin_unlock_irq(&sem->wait_lock); |
a7d2c573 ML |
228 | |
229 | /* Block until there are no active lockers. */ | |
230 | do { | |
231 | schedule(); | |
232 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
9b0fc9c0 | 233 | } while ((count = sem->count) & RWSEM_ACTIVE_MASK); |
a7d2c573 | 234 | |
023fe4f7 | 235 | raw_spin_lock_irq(&sem->wait_lock); |
1e78277c ML |
236 | } |
237 | ||
023fe4f7 ML |
238 | list_del(&waiter.list); |
239 | raw_spin_unlock_irq(&sem->wait_lock); | |
1e78277c ML |
240 | tsk->state = TASK_RUNNING; |
241 | ||
242 | return sem; | |
1da177e4 LT |
243 | } |
244 | ||
245 | /* | |
246 | * handle waking up a waiter on the semaphore | |
247 | * - up_read/up_write has decremented the active part of count if we come here | |
248 | */ | |
d1233754 | 249 | struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) |
1da177e4 LT |
250 | { |
251 | unsigned long flags; | |
252 | ||
ddb6c9b5 | 253 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
1da177e4 LT |
254 | |
255 | /* do nothing if list empty */ | |
256 | if (!list_empty(&sem->wait_list)) | |
70bdc6e0 | 257 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
1da177e4 | 258 | |
ddb6c9b5 | 259 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
1da177e4 | 260 | |
1da177e4 LT |
261 | return sem; |
262 | } | |
263 | ||
264 | /* | |
265 | * downgrade a write lock into a read lock | |
266 | * - caller incremented waiting part of count and discovered it still negative | |
267 | * - just wake up any readers at the front of the queue | |
268 | */ | |
d1233754 | 269 | struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) |
1da177e4 LT |
270 | { |
271 | unsigned long flags; | |
272 | ||
ddb6c9b5 | 273 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
1da177e4 LT |
274 | |
275 | /* do nothing if list empty */ | |
276 | if (!list_empty(&sem->wait_list)) | |
70bdc6e0 | 277 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
1da177e4 | 278 | |
ddb6c9b5 | 279 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
1da177e4 | 280 | |
1da177e4 LT |
281 | return sem; |
282 | } | |
283 | ||
284 | EXPORT_SYMBOL(rwsem_down_read_failed); | |
285 | EXPORT_SYMBOL(rwsem_down_write_failed); | |
286 | EXPORT_SYMBOL(rwsem_wake); | |
287 | EXPORT_SYMBOL(rwsem_downgrade_wake); |