| 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 |
| 5 | * |
| 6 | * Writer lock-stealing by Alex Shi <alex.shi@intel.com> |
| 7 | * and Michel Lespinasse <walken@google.com> |
| 8 | * |
| 9 | * Optimistic spinning by Tim Chen <tim.c.chen@intel.com> |
| 10 | * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes. |
| 11 | */ |
| 12 | #include <linux/rwsem.h> |
| 13 | #include <linux/sched.h> |
| 14 | #include <linux/init.h> |
| 15 | #include <linux/export.h> |
| 16 | #include <linux/sched/rt.h> |
| 17 | #include <linux/osq_lock.h> |
| 18 | |
| 19 | #include "rwsem.h" |
| 20 | |
| 21 | /* |
| 22 | * Guide to the rw_semaphore's count field for common values. |
| 23 | * (32-bit case illustrated, similar for 64-bit) |
| 24 | * |
| 25 | * 0x0000000X (1) X readers active or attempting lock, no writer waiting |
| 26 | * X = #active_readers + #readers attempting to lock |
| 27 | * (X*ACTIVE_BIAS) |
| 28 | * |
| 29 | * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or |
| 30 | * attempting to read lock or write lock. |
| 31 | * |
| 32 | * 0xffff000X (1) X readers active or attempting lock, with waiters for lock |
| 33 | * X = #active readers + # readers attempting lock |
| 34 | * (X*ACTIVE_BIAS + WAITING_BIAS) |
| 35 | * (2) 1 writer attempting lock, no waiters for lock |
| 36 | * X-1 = #active readers + #readers attempting lock |
| 37 | * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS) |
| 38 | * (3) 1 writer active, no waiters for lock |
| 39 | * X-1 = #active readers + #readers attempting lock |
| 40 | * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS) |
| 41 | * |
| 42 | * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock |
| 43 | * (WAITING_BIAS + ACTIVE_BIAS) |
| 44 | * (2) 1 writer active or attempting lock, no waiters for lock |
| 45 | * (ACTIVE_WRITE_BIAS) |
| 46 | * |
| 47 | * 0xffff0000 (1) There are writers or readers queued but none active |
| 48 | * or in the process of attempting lock. |
| 49 | * (WAITING_BIAS) |
| 50 | * Note: writer can attempt to steal lock for this count by adding |
| 51 | * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count |
| 52 | * |
| 53 | * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue. |
| 54 | * (ACTIVE_WRITE_BIAS + WAITING_BIAS) |
| 55 | * |
| 56 | * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking |
| 57 | * the count becomes more than 0 for successful lock acquisition, |
| 58 | * i.e. the case where there are only readers or nobody has lock. |
| 59 | * (1st and 2nd case above). |
| 60 | * |
| 61 | * Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and |
| 62 | * checking the count becomes ACTIVE_WRITE_BIAS for successful lock |
| 63 | * acquisition (i.e. nobody else has lock or attempts lock). If |
| 64 | * unsuccessful, in rwsem_down_write_failed, we'll check to see if there |
| 65 | * are only waiters but none active (5th case above), and attempt to |
| 66 | * steal the lock. |
| 67 | * |
| 68 | */ |
| 69 | |
| 70 | /* |
| 71 | * Initialize an rwsem: |
| 72 | */ |
| 73 | void __init_rwsem(struct rw_semaphore *sem, const char *name, |
| 74 | struct lock_class_key *key) |
| 75 | { |
| 76 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
| 77 | /* |
| 78 | * Make sure we are not reinitializing a held semaphore: |
| 79 | */ |
| 80 | debug_check_no_locks_freed((void *)sem, sizeof(*sem)); |
| 81 | lockdep_init_map(&sem->dep_map, name, key, 0); |
| 82 | #endif |
| 83 | sem->count = RWSEM_UNLOCKED_VALUE; |
| 84 | raw_spin_lock_init(&sem->wait_lock); |
| 85 | INIT_LIST_HEAD(&sem->wait_list); |
| 86 | #ifdef CONFIG_RWSEM_SPIN_ON_OWNER |
| 87 | sem->owner = NULL; |
| 88 | osq_lock_init(&sem->osq); |
| 89 | #endif |
| 90 | } |
| 91 | |
| 92 | EXPORT_SYMBOL(__init_rwsem); |
| 93 | |
| 94 | enum rwsem_waiter_type { |
| 95 | RWSEM_WAITING_FOR_WRITE, |
| 96 | RWSEM_WAITING_FOR_READ |
| 97 | }; |
| 98 | |
| 99 | struct rwsem_waiter { |
| 100 | struct list_head list; |
| 101 | struct task_struct *task; |
| 102 | enum rwsem_waiter_type type; |
| 103 | }; |
| 104 | |
| 105 | enum rwsem_wake_type { |
| 106 | RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */ |
| 107 | RWSEM_WAKE_READERS, /* Wake readers only */ |
| 108 | RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */ |
| 109 | }; |
| 110 | |
| 111 | /* |
| 112 | * handle the lock release when processes blocked on it that can now run |
| 113 | * - if we come here from up_xxxx(), then: |
| 114 | * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) |
| 115 | * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) |
| 116 | * - there must be someone on the queue |
| 117 | * - the spinlock must be held by the caller |
| 118 | * - woken process blocks are discarded from the list after having task zeroed |
| 119 | * - writers are only woken if downgrading is false |
| 120 | */ |
| 121 | static struct rw_semaphore * |
| 122 | __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) |
| 123 | { |
| 124 | struct rwsem_waiter *waiter; |
| 125 | struct task_struct *tsk; |
| 126 | struct list_head *next; |
| 127 | long oldcount, woken, loop, adjustment; |
| 128 | |
| 129 | waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); |
| 130 | if (waiter->type == RWSEM_WAITING_FOR_WRITE) { |
| 131 | if (wake_type == RWSEM_WAKE_ANY) |
| 132 | /* Wake writer at the front of the queue, but do not |
| 133 | * grant it the lock yet as we want other writers |
| 134 | * to be able to steal it. Readers, on the other hand, |
| 135 | * will block as they will notice the queued writer. |
| 136 | */ |
| 137 | wake_up_process(waiter->task); |
| 138 | goto out; |
| 139 | } |
| 140 | |
| 141 | /* Writers might steal the lock before we grant it to the next reader. |
| 142 | * We prefer to do the first reader grant before counting readers |
| 143 | * so we can bail out early if a writer stole the lock. |
| 144 | */ |
| 145 | adjustment = 0; |
| 146 | if (wake_type != RWSEM_WAKE_READ_OWNED) { |
| 147 | adjustment = RWSEM_ACTIVE_READ_BIAS; |
| 148 | try_reader_grant: |
| 149 | oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; |
| 150 | if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { |
| 151 | /* A writer stole the lock. Undo our reader grant. */ |
| 152 | if (rwsem_atomic_update(-adjustment, sem) & |
| 153 | RWSEM_ACTIVE_MASK) |
| 154 | goto out; |
| 155 | /* Last active locker left. Retry waking readers. */ |
| 156 | goto try_reader_grant; |
| 157 | } |
| 158 | } |
| 159 | |
| 160 | /* Grant an infinite number of read locks to the readers at the front |
| 161 | * of the queue. Note we increment the 'active part' of the count by |
| 162 | * the number of readers before waking any processes up. |
| 163 | */ |
| 164 | woken = 0; |
| 165 | do { |
| 166 | woken++; |
| 167 | |
| 168 | if (waiter->list.next == &sem->wait_list) |
| 169 | break; |
| 170 | |
| 171 | waiter = list_entry(waiter->list.next, |
| 172 | struct rwsem_waiter, list); |
| 173 | |
| 174 | } while (waiter->type != RWSEM_WAITING_FOR_WRITE); |
| 175 | |
| 176 | adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; |
| 177 | if (waiter->type != RWSEM_WAITING_FOR_WRITE) |
| 178 | /* hit end of list above */ |
| 179 | adjustment -= RWSEM_WAITING_BIAS; |
| 180 | |
| 181 | if (adjustment) |
| 182 | rwsem_atomic_add(adjustment, sem); |
| 183 | |
| 184 | next = sem->wait_list.next; |
| 185 | loop = woken; |
| 186 | do { |
| 187 | waiter = list_entry(next, struct rwsem_waiter, list); |
| 188 | next = waiter->list.next; |
| 189 | tsk = waiter->task; |
| 190 | /* |
| 191 | * Make sure we do not wakeup the next reader before |
| 192 | * setting the nil condition to grant the next reader; |
| 193 | * otherwise we could miss the wakeup on the other |
| 194 | * side and end up sleeping again. See the pairing |
| 195 | * in rwsem_down_read_failed(). |
| 196 | */ |
| 197 | smp_mb(); |
| 198 | waiter->task = NULL; |
| 199 | wake_up_process(tsk); |
| 200 | put_task_struct(tsk); |
| 201 | } while (--loop); |
| 202 | |
| 203 | sem->wait_list.next = next; |
| 204 | next->prev = &sem->wait_list; |
| 205 | |
| 206 | out: |
| 207 | return sem; |
| 208 | } |
| 209 | |
| 210 | /* |
| 211 | * Wait for the read lock to be granted |
| 212 | */ |
| 213 | __visible |
| 214 | struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) |
| 215 | { |
| 216 | long count, adjustment = -RWSEM_ACTIVE_READ_BIAS; |
| 217 | struct rwsem_waiter waiter; |
| 218 | struct task_struct *tsk = current; |
| 219 | |
| 220 | /* set up my own style of waitqueue */ |
| 221 | waiter.task = tsk; |
| 222 | waiter.type = RWSEM_WAITING_FOR_READ; |
| 223 | get_task_struct(tsk); |
| 224 | |
| 225 | raw_spin_lock_irq(&sem->wait_lock); |
| 226 | if (list_empty(&sem->wait_list)) |
| 227 | adjustment += RWSEM_WAITING_BIAS; |
| 228 | list_add_tail(&waiter.list, &sem->wait_list); |
| 229 | |
| 230 | /* we're now waiting on the lock, but no longer actively locking */ |
| 231 | count = rwsem_atomic_update(adjustment, sem); |
| 232 | |
| 233 | /* If there are no active locks, wake the front queued process(es). |
| 234 | * |
| 235 | * If there are no writers and we are first in the queue, |
| 236 | * wake our own waiter to join the existing active readers ! |
| 237 | */ |
| 238 | if (count == RWSEM_WAITING_BIAS || |
| 239 | (count > RWSEM_WAITING_BIAS && |
| 240 | adjustment != -RWSEM_ACTIVE_READ_BIAS)) |
| 241 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
| 242 | |
| 243 | raw_spin_unlock_irq(&sem->wait_lock); |
| 244 | |
| 245 | /* wait to be given the lock */ |
| 246 | while (true) { |
| 247 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); |
| 248 | if (!waiter.task) |
| 249 | break; |
| 250 | schedule(); |
| 251 | } |
| 252 | |
| 253 | __set_task_state(tsk, TASK_RUNNING); |
| 254 | return sem; |
| 255 | } |
| 256 | EXPORT_SYMBOL(rwsem_down_read_failed); |
| 257 | |
| 258 | static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) |
| 259 | { |
| 260 | /* |
| 261 | * Try acquiring the write lock. Check count first in order |
| 262 | * to reduce unnecessary expensive cmpxchg() operations. |
| 263 | */ |
| 264 | if (count == RWSEM_WAITING_BIAS && |
| 265 | cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, |
| 266 | RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) { |
| 267 | if (!list_is_singular(&sem->wait_list)) |
| 268 | rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); |
| 269 | rwsem_set_owner(sem); |
| 270 | return true; |
| 271 | } |
| 272 | |
| 273 | return false; |
| 274 | } |
| 275 | |
| 276 | #ifdef CONFIG_RWSEM_SPIN_ON_OWNER |
| 277 | /* |
| 278 | * Try to acquire write lock before the writer has been put on wait queue. |
| 279 | */ |
| 280 | static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) |
| 281 | { |
| 282 | long old, count = READ_ONCE(sem->count); |
| 283 | |
| 284 | while (true) { |
| 285 | if (!(count == 0 || count == RWSEM_WAITING_BIAS)) |
| 286 | return false; |
| 287 | |
| 288 | old = cmpxchg_acquire(&sem->count, count, |
| 289 | count + RWSEM_ACTIVE_WRITE_BIAS); |
| 290 | if (old == count) { |
| 291 | rwsem_set_owner(sem); |
| 292 | return true; |
| 293 | } |
| 294 | |
| 295 | count = old; |
| 296 | } |
| 297 | } |
| 298 | |
| 299 | static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) |
| 300 | { |
| 301 | struct task_struct *owner; |
| 302 | bool ret = true; |
| 303 | |
| 304 | if (need_resched()) |
| 305 | return false; |
| 306 | |
| 307 | rcu_read_lock(); |
| 308 | owner = READ_ONCE(sem->owner); |
| 309 | if (!owner) { |
| 310 | long count = READ_ONCE(sem->count); |
| 311 | /* |
| 312 | * If sem->owner is not set, yet we have just recently entered the |
| 313 | * slowpath with the lock being active, then there is a possibility |
| 314 | * reader(s) may have the lock. To be safe, bail spinning in these |
| 315 | * situations. |
| 316 | */ |
| 317 | if (count & RWSEM_ACTIVE_MASK) |
| 318 | ret = false; |
| 319 | goto done; |
| 320 | } |
| 321 | |
| 322 | ret = owner->on_cpu; |
| 323 | done: |
| 324 | rcu_read_unlock(); |
| 325 | return ret; |
| 326 | } |
| 327 | |
| 328 | static noinline |
| 329 | bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner) |
| 330 | { |
| 331 | long count; |
| 332 | |
| 333 | rcu_read_lock(); |
| 334 | while (sem->owner == owner) { |
| 335 | /* |
| 336 | * Ensure we emit the owner->on_cpu, dereference _after_ |
| 337 | * checking sem->owner still matches owner, if that fails, |
| 338 | * owner might point to free()d memory, if it still matches, |
| 339 | * the rcu_read_lock() ensures the memory stays valid. |
| 340 | */ |
| 341 | barrier(); |
| 342 | |
| 343 | /* abort spinning when need_resched or owner is not running */ |
| 344 | if (!owner->on_cpu || need_resched()) { |
| 345 | rcu_read_unlock(); |
| 346 | return false; |
| 347 | } |
| 348 | |
| 349 | cpu_relax_lowlatency(); |
| 350 | } |
| 351 | rcu_read_unlock(); |
| 352 | |
| 353 | if (READ_ONCE(sem->owner)) |
| 354 | return true; /* new owner, continue spinning */ |
| 355 | |
| 356 | /* |
| 357 | * When the owner is not set, the lock could be free or |
| 358 | * held by readers. Check the counter to verify the |
| 359 | * state. |
| 360 | */ |
| 361 | count = READ_ONCE(sem->count); |
| 362 | return (count == 0 || count == RWSEM_WAITING_BIAS); |
| 363 | } |
| 364 | |
| 365 | static bool rwsem_optimistic_spin(struct rw_semaphore *sem) |
| 366 | { |
| 367 | struct task_struct *owner; |
| 368 | bool taken = false; |
| 369 | |
| 370 | preempt_disable(); |
| 371 | |
| 372 | /* sem->wait_lock should not be held when doing optimistic spinning */ |
| 373 | if (!rwsem_can_spin_on_owner(sem)) |
| 374 | goto done; |
| 375 | |
| 376 | if (!osq_lock(&sem->osq)) |
| 377 | goto done; |
| 378 | |
| 379 | while (true) { |
| 380 | owner = READ_ONCE(sem->owner); |
| 381 | if (owner && !rwsem_spin_on_owner(sem, owner)) |
| 382 | break; |
| 383 | |
| 384 | /* wait_lock will be acquired if write_lock is obtained */ |
| 385 | if (rwsem_try_write_lock_unqueued(sem)) { |
| 386 | taken = true; |
| 387 | break; |
| 388 | } |
| 389 | |
| 390 | /* |
| 391 | * When there's no owner, we might have preempted between the |
| 392 | * owner acquiring the lock and setting the owner field. If |
| 393 | * we're an RT task that will live-lock because we won't let |
| 394 | * the owner complete. |
| 395 | */ |
| 396 | if (!owner && (need_resched() || rt_task(current))) |
| 397 | break; |
| 398 | |
| 399 | /* |
| 400 | * The cpu_relax() call is a compiler barrier which forces |
| 401 | * everything in this loop to be re-loaded. We don't need |
| 402 | * memory barriers as we'll eventually observe the right |
| 403 | * values at the cost of a few extra spins. |
| 404 | */ |
| 405 | cpu_relax_lowlatency(); |
| 406 | } |
| 407 | osq_unlock(&sem->osq); |
| 408 | done: |
| 409 | preempt_enable(); |
| 410 | return taken; |
| 411 | } |
| 412 | |
| 413 | /* |
| 414 | * Return true if the rwsem has active spinner |
| 415 | */ |
| 416 | static inline bool rwsem_has_spinner(struct rw_semaphore *sem) |
| 417 | { |
| 418 | return osq_is_locked(&sem->osq); |
| 419 | } |
| 420 | |
| 421 | #else |
| 422 | static bool rwsem_optimistic_spin(struct rw_semaphore *sem) |
| 423 | { |
| 424 | return false; |
| 425 | } |
| 426 | |
| 427 | static inline bool rwsem_has_spinner(struct rw_semaphore *sem) |
| 428 | { |
| 429 | return false; |
| 430 | } |
| 431 | #endif |
| 432 | |
| 433 | /* |
| 434 | * Wait until we successfully acquire the write lock |
| 435 | */ |
| 436 | __visible |
| 437 | struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) |
| 438 | { |
| 439 | long count; |
| 440 | bool waiting = true; /* any queued threads before us */ |
| 441 | struct rwsem_waiter waiter; |
| 442 | |
| 443 | /* undo write bias from down_write operation, stop active locking */ |
| 444 | count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem); |
| 445 | |
| 446 | /* do optimistic spinning and steal lock if possible */ |
| 447 | if (rwsem_optimistic_spin(sem)) |
| 448 | return sem; |
| 449 | |
| 450 | /* |
| 451 | * Optimistic spinning failed, proceed to the slowpath |
| 452 | * and block until we can acquire the sem. |
| 453 | */ |
| 454 | waiter.task = current; |
| 455 | waiter.type = RWSEM_WAITING_FOR_WRITE; |
| 456 | |
| 457 | raw_spin_lock_irq(&sem->wait_lock); |
| 458 | |
| 459 | /* account for this before adding a new element to the list */ |
| 460 | if (list_empty(&sem->wait_list)) |
| 461 | waiting = false; |
| 462 | |
| 463 | list_add_tail(&waiter.list, &sem->wait_list); |
| 464 | |
| 465 | /* we're now waiting on the lock, but no longer actively locking */ |
| 466 | if (waiting) { |
| 467 | count = READ_ONCE(sem->count); |
| 468 | |
| 469 | /* |
| 470 | * If there were already threads queued before us and there are |
| 471 | * no active writers, the lock must be read owned; so we try to |
| 472 | * wake any read locks that were queued ahead of us. |
| 473 | */ |
| 474 | if (count > RWSEM_WAITING_BIAS) |
| 475 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS); |
| 476 | |
| 477 | } else |
| 478 | count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); |
| 479 | |
| 480 | /* wait until we successfully acquire the lock */ |
| 481 | set_current_state(TASK_UNINTERRUPTIBLE); |
| 482 | while (true) { |
| 483 | if (rwsem_try_write_lock(count, sem)) |
| 484 | break; |
| 485 | raw_spin_unlock_irq(&sem->wait_lock); |
| 486 | |
| 487 | /* Block until there are no active lockers. */ |
| 488 | do { |
| 489 | schedule(); |
| 490 | set_current_state(TASK_UNINTERRUPTIBLE); |
| 491 | } while ((count = sem->count) & RWSEM_ACTIVE_MASK); |
| 492 | |
| 493 | raw_spin_lock_irq(&sem->wait_lock); |
| 494 | } |
| 495 | __set_current_state(TASK_RUNNING); |
| 496 | |
| 497 | list_del(&waiter.list); |
| 498 | raw_spin_unlock_irq(&sem->wait_lock); |
| 499 | |
| 500 | return sem; |
| 501 | } |
| 502 | EXPORT_SYMBOL(rwsem_down_write_failed); |
| 503 | |
| 504 | /* |
| 505 | * handle waking up a waiter on the semaphore |
| 506 | * - up_read/up_write has decremented the active part of count if we come here |
| 507 | */ |
| 508 | __visible |
| 509 | struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) |
| 510 | { |
| 511 | unsigned long flags; |
| 512 | |
| 513 | /* |
| 514 | * If a spinner is present, it is not necessary to do the wakeup. |
| 515 | * Try to do wakeup only if the trylock succeeds to minimize |
| 516 | * spinlock contention which may introduce too much delay in the |
| 517 | * unlock operation. |
| 518 | * |
| 519 | * spinning writer up_write/up_read caller |
| 520 | * --------------- ----------------------- |
| 521 | * [S] osq_unlock() [L] osq |
| 522 | * MB RMB |
| 523 | * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock) |
| 524 | * |
| 525 | * Here, it is important to make sure that there won't be a missed |
| 526 | * wakeup while the rwsem is free and the only spinning writer goes |
| 527 | * to sleep without taking the rwsem. Even when the spinning writer |
| 528 | * is just going to break out of the waiting loop, it will still do |
| 529 | * a trylock in rwsem_down_write_failed() before sleeping. IOW, if |
| 530 | * rwsem_has_spinner() is true, it will guarantee at least one |
| 531 | * trylock attempt on the rwsem later on. |
| 532 | */ |
| 533 | if (rwsem_has_spinner(sem)) { |
| 534 | /* |
| 535 | * The smp_rmb() here is to make sure that the spinner |
| 536 | * state is consulted before reading the wait_lock. |
| 537 | */ |
| 538 | smp_rmb(); |
| 539 | if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags)) |
| 540 | return sem; |
| 541 | goto locked; |
| 542 | } |
| 543 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
| 544 | locked: |
| 545 | |
| 546 | /* do nothing if list empty */ |
| 547 | if (!list_empty(&sem->wait_list)) |
| 548 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
| 549 | |
| 550 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
| 551 | |
| 552 | return sem; |
| 553 | } |
| 554 | EXPORT_SYMBOL(rwsem_wake); |
| 555 | |
| 556 | /* |
| 557 | * downgrade a write lock into a read lock |
| 558 | * - caller incremented waiting part of count and discovered it still negative |
| 559 | * - just wake up any readers at the front of the queue |
| 560 | */ |
| 561 | __visible |
| 562 | struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) |
| 563 | { |
| 564 | unsigned long flags; |
| 565 | |
| 566 | raw_spin_lock_irqsave(&sem->wait_lock, flags); |
| 567 | |
| 568 | /* do nothing if list empty */ |
| 569 | if (!list_empty(&sem->wait_list)) |
| 570 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
| 571 | |
| 572 | raw_spin_unlock_irqrestore(&sem->wait_lock, flags); |
| 573 | |
| 574 | return sem; |
| 575 | } |
| 576 | EXPORT_SYMBOL(rwsem_downgrade_wake); |