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1 : : // SPDX-License-Identifier: GPL-2.0
2 : : /* kernel/rwsem.c: R/W semaphores, public implementation
3 : : *
4 : : * Written by David Howells (dhowells@redhat.com).
5 : : * Derived from asm-i386/semaphore.h
6 : : *
7 : : * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
8 : : * and Michel Lespinasse <walken@google.com>
9 : : *
10 : : * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
11 : : * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
12 : : *
13 : : * Rwsem count bit fields re-definition and rwsem rearchitecture by
14 : : * Waiman Long <longman@redhat.com> and
15 : : * Peter Zijlstra <peterz@infradead.org>.
16 : : */
17 : :
18 : : #include <linux/types.h>
19 : : #include <linux/kernel.h>
20 : : #include <linux/sched.h>
21 : : #include <linux/sched/rt.h>
22 : : #include <linux/sched/task.h>
23 : : #include <linux/sched/debug.h>
24 : : #include <linux/sched/wake_q.h>
25 : : #include <linux/sched/signal.h>
26 : : #include <linux/sched/clock.h>
27 : : #include <linux/export.h>
28 : : #include <linux/rwsem.h>
29 : : #include <linux/atomic.h>
30 : :
31 : : #include "rwsem.h"
32 : : #include "lock_events.h"
33 : :
34 : : /*
35 : : * The least significant 3 bits of the owner value has the following
36 : : * meanings when set.
37 : : * - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
38 : : * - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
39 : : * - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.
40 : : *
41 : : * When the rwsem is either owned by an anonymous writer, or it is
42 : : * reader-owned, but a spinning writer has timed out, both nonspinnable
43 : : * bits will be set to disable optimistic spinning by readers and writers.
44 : : * In the later case, the last unlocking reader should then check the
45 : : * writer nonspinnable bit and clear it only to give writers preference
46 : : * to acquire the lock via optimistic spinning, but not readers. Similar
47 : : * action is also done in the reader slowpath.
48 : :
49 : : * When a writer acquires a rwsem, it puts its task_struct pointer
50 : : * into the owner field. It is cleared after an unlock.
51 : : *
52 : : * When a reader acquires a rwsem, it will also puts its task_struct
53 : : * pointer into the owner field with the RWSEM_READER_OWNED bit set.
54 : : * On unlock, the owner field will largely be left untouched. So
55 : : * for a free or reader-owned rwsem, the owner value may contain
56 : : * information about the last reader that acquires the rwsem.
57 : : *
58 : : * That information may be helpful in debugging cases where the system
59 : : * seems to hang on a reader owned rwsem especially if only one reader
60 : : * is involved. Ideally we would like to track all the readers that own
61 : : * a rwsem, but the overhead is simply too big.
62 : : *
63 : : * Reader optimistic spinning is helpful when the reader critical section
64 : : * is short and there aren't that many readers around. It makes readers
65 : : * relatively more preferred than writers. When a writer times out spinning
66 : : * on a reader-owned lock and set the nospinnable bits, there are two main
67 : : * reasons for that.
68 : : *
69 : : * 1) The reader critical section is long, perhaps the task sleeps after
70 : : * acquiring the read lock.
71 : : * 2) There are just too many readers contending the lock causing it to
72 : : * take a while to service all of them.
73 : : *
74 : : * In the former case, long reader critical section will impede the progress
75 : : * of writers which is usually more important for system performance. In
76 : : * the later case, reader optimistic spinning tends to make the reader
77 : : * groups that contain readers that acquire the lock together smaller
78 : : * leading to more of them. That may hurt performance in some cases. In
79 : : * other words, the setting of nonspinnable bits indicates that reader
80 : : * optimistic spinning may not be helpful for those workloads that cause
81 : : * it.
82 : : *
83 : : * Therefore, any writers that had observed the setting of the writer
84 : : * nonspinnable bit for a given rwsem after they fail to acquire the lock
85 : : * via optimistic spinning will set the reader nonspinnable bit once they
86 : : * acquire the write lock. Similarly, readers that observe the setting
87 : : * of reader nonspinnable bit at slowpath entry will set the reader
88 : : * nonspinnable bits when they acquire the read lock via the wakeup path.
89 : : *
90 : : * Once the reader nonspinnable bit is on, it will only be reset when
91 : : * a writer is able to acquire the rwsem in the fast path or somehow a
92 : : * reader or writer in the slowpath doesn't observe the nonspinable bit.
93 : : *
94 : : * This is to discourage reader optmistic spinning on that particular
95 : : * rwsem and make writers more preferred. This adaptive disabling of reader
96 : : * optimistic spinning will alleviate the negative side effect of this
97 : : * feature.
98 : : */
99 : : #define RWSEM_READER_OWNED (1UL << 0)
100 : : #define RWSEM_RD_NONSPINNABLE (1UL << 1)
101 : : #define RWSEM_WR_NONSPINNABLE (1UL << 2)
102 : : #define RWSEM_NONSPINNABLE (RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)
103 : : #define RWSEM_OWNER_FLAGS_MASK (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
104 : :
105 : : #ifdef CONFIG_DEBUG_RWSEMS
106 : : # define DEBUG_RWSEMS_WARN_ON(c, sem) do { \
107 : : if (!debug_locks_silent && \
108 : : WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
109 : : #c, atomic_long_read(&(sem)->count), \
110 : : (unsigned long) sem->magic, \
111 : : atomic_long_read(&(sem)->owner), (long)current, \
112 : : list_empty(&(sem)->wait_list) ? "" : "not ")) \
113 : : debug_locks_off(); \
114 : : } while (0)
115 : : #else
116 : : # define DEBUG_RWSEMS_WARN_ON(c, sem)
117 : : #endif
118 : :
119 : : /*
120 : : * On 64-bit architectures, the bit definitions of the count are:
121 : : *
122 : : * Bit 0 - writer locked bit
123 : : * Bit 1 - waiters present bit
124 : : * Bit 2 - lock handoff bit
125 : : * Bits 3-7 - reserved
126 : : * Bits 8-62 - 55-bit reader count
127 : : * Bit 63 - read fail bit
128 : : *
129 : : * On 32-bit architectures, the bit definitions of the count are:
130 : : *
131 : : * Bit 0 - writer locked bit
132 : : * Bit 1 - waiters present bit
133 : : * Bit 2 - lock handoff bit
134 : : * Bits 3-7 - reserved
135 : : * Bits 8-30 - 23-bit reader count
136 : : * Bit 31 - read fail bit
137 : : *
138 : : * It is not likely that the most significant bit (read fail bit) will ever
139 : : * be set. This guard bit is still checked anyway in the down_read() fastpath
140 : : * just in case we need to use up more of the reader bits for other purpose
141 : : * in the future.
142 : : *
143 : : * atomic_long_fetch_add() is used to obtain reader lock, whereas
144 : : * atomic_long_cmpxchg() will be used to obtain writer lock.
145 : : *
146 : : * There are three places where the lock handoff bit may be set or cleared.
147 : : * 1) rwsem_mark_wake() for readers.
148 : : * 2) rwsem_try_write_lock() for writers.
149 : : * 3) Error path of rwsem_down_write_slowpath().
150 : : *
151 : : * For all the above cases, wait_lock will be held. A writer must also
152 : : * be the first one in the wait_list to be eligible for setting the handoff
153 : : * bit. So concurrent setting/clearing of handoff bit is not possible.
154 : : */
155 : : #define RWSEM_WRITER_LOCKED (1UL << 0)
156 : : #define RWSEM_FLAG_WAITERS (1UL << 1)
157 : : #define RWSEM_FLAG_HANDOFF (1UL << 2)
158 : : #define RWSEM_FLAG_READFAIL (1UL << (BITS_PER_LONG - 1))
159 : :
160 : : #define RWSEM_READER_SHIFT 8
161 : : #define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
162 : : #define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
163 : : #define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
164 : : #define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
165 : : #define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
166 : : RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
167 : :
168 : : /*
169 : : * All writes to owner are protected by WRITE_ONCE() to make sure that
170 : : * store tearing can't happen as optimistic spinners may read and use
171 : : * the owner value concurrently without lock. Read from owner, however,
172 : : * may not need READ_ONCE() as long as the pointer value is only used
173 : : * for comparison and isn't being dereferenced.
174 : : */
175 : : static inline void rwsem_set_owner(struct rw_semaphore *sem)
176 : : {
177 : 3 : atomic_long_set(&sem->owner, (long)current);
178 : : }
179 : :
180 : : static inline void rwsem_clear_owner(struct rw_semaphore *sem)
181 : : {
182 : : atomic_long_set(&sem->owner, 0);
183 : : }
184 : :
185 : : /*
186 : : * Test the flags in the owner field.
187 : : */
188 : : static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
189 : : {
190 : 3 : return atomic_long_read(&sem->owner) & flags;
191 : : }
192 : :
193 : : /*
194 : : * The task_struct pointer of the last owning reader will be left in
195 : : * the owner field.
196 : : *
197 : : * Note that the owner value just indicates the task has owned the rwsem
198 : : * previously, it may not be the real owner or one of the real owners
199 : : * anymore when that field is examined, so take it with a grain of salt.
200 : : *
201 : : * The reader non-spinnable bit is preserved.
202 : : */
203 : : static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
204 : : struct task_struct *owner)
205 : : {
206 : 3 : unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
207 : 3 : (atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);
208 : :
209 : : atomic_long_set(&sem->owner, val);
210 : : }
211 : :
212 : : static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
213 : : {
214 : 3 : __rwsem_set_reader_owned(sem, current);
215 : : }
216 : :
217 : : /*
218 : : * Return true if the rwsem is owned by a reader.
219 : : */
220 : : static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
221 : : {
222 : : #ifdef CONFIG_DEBUG_RWSEMS
223 : : /*
224 : : * Check the count to see if it is write-locked.
225 : : */
226 : : long count = atomic_long_read(&sem->count);
227 : :
228 : : if (count & RWSEM_WRITER_MASK)
229 : : return false;
230 : : #endif
231 : : return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
232 : : }
233 : :
234 : : #ifdef CONFIG_DEBUG_RWSEMS
235 : : /*
236 : : * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
237 : : * is a task pointer in owner of a reader-owned rwsem, it will be the
238 : : * real owner or one of the real owners. The only exception is when the
239 : : * unlock is done by up_read_non_owner().
240 : : */
241 : : static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
242 : : {
243 : : unsigned long val = atomic_long_read(&sem->owner);
244 : :
245 : : while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
246 : : if (atomic_long_try_cmpxchg(&sem->owner, &val,
247 : : val & RWSEM_OWNER_FLAGS_MASK))
248 : : return;
249 : : }
250 : : }
251 : : #else
252 : : static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
253 : : {
254 : : }
255 : : #endif
256 : :
257 : : /*
258 : : * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
259 : : * remains set. Otherwise, the operation will be aborted.
260 : : */
261 : 3 : static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
262 : : {
263 : 3 : unsigned long owner = atomic_long_read(&sem->owner);
264 : :
265 : : do {
266 : 3 : if (!(owner & RWSEM_READER_OWNED))
267 : : break;
268 : 3 : if (owner & RWSEM_NONSPINNABLE)
269 : : break;
270 : 3 : } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
271 : 3 : owner | RWSEM_NONSPINNABLE));
272 : 3 : }
273 : :
274 : 3 : static inline bool rwsem_read_trylock(struct rw_semaphore *sem)
275 : : {
276 : 3 : long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
277 : 3 : if (WARN_ON_ONCE(cnt < 0))
278 : 0 : rwsem_set_nonspinnable(sem);
279 : 3 : return !(cnt & RWSEM_READ_FAILED_MASK);
280 : : }
281 : :
282 : : /*
283 : : * Return just the real task structure pointer of the owner
284 : : */
285 : : static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
286 : : {
287 : : return (struct task_struct *)
288 : : (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
289 : : }
290 : :
291 : : /*
292 : : * Return the real task structure pointer of the owner and the embedded
293 : : * flags in the owner. pflags must be non-NULL.
294 : : */
295 : : static inline struct task_struct *
296 : : rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
297 : : {
298 : : unsigned long owner = atomic_long_read(&sem->owner);
299 : :
300 : 3 : *pflags = owner & RWSEM_OWNER_FLAGS_MASK;
301 : 3 : return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
302 : : }
303 : :
304 : : /*
305 : : * Guide to the rw_semaphore's count field.
306 : : *
307 : : * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
308 : : * by a writer.
309 : : *
310 : : * The lock is owned by readers when
311 : : * (1) the RWSEM_WRITER_LOCKED isn't set in count,
312 : : * (2) some of the reader bits are set in count, and
313 : : * (3) the owner field has RWSEM_READ_OWNED bit set.
314 : : *
315 : : * Having some reader bits set is not enough to guarantee a readers owned
316 : : * lock as the readers may be in the process of backing out from the count
317 : : * and a writer has just released the lock. So another writer may steal
318 : : * the lock immediately after that.
319 : : */
320 : :
321 : : /*
322 : : * Initialize an rwsem:
323 : : */
324 : 3 : void __init_rwsem(struct rw_semaphore *sem, const char *name,
325 : : struct lock_class_key *key)
326 : : {
327 : : #ifdef CONFIG_DEBUG_LOCK_ALLOC
328 : : /*
329 : : * Make sure we are not reinitializing a held semaphore:
330 : : */
331 : : debug_check_no_locks_freed((void *)sem, sizeof(*sem));
332 : : lockdep_init_map(&sem->dep_map, name, key, 0);
333 : : #endif
334 : : #ifdef CONFIG_DEBUG_RWSEMS
335 : : sem->magic = sem;
336 : : #endif
337 : : atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
338 : 3 : raw_spin_lock_init(&sem->wait_lock);
339 : 3 : INIT_LIST_HEAD(&sem->wait_list);
340 : : atomic_long_set(&sem->owner, 0L);
341 : : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
342 : : osq_lock_init(&sem->osq);
343 : : #endif
344 : 3 : }
345 : : EXPORT_SYMBOL(__init_rwsem);
346 : :
347 : : enum rwsem_waiter_type {
348 : : RWSEM_WAITING_FOR_WRITE,
349 : : RWSEM_WAITING_FOR_READ
350 : : };
351 : :
352 : : struct rwsem_waiter {
353 : : struct list_head list;
354 : : struct task_struct *task;
355 : : enum rwsem_waiter_type type;
356 : : unsigned long timeout;
357 : : unsigned long last_rowner;
358 : : };
359 : : #define rwsem_first_waiter(sem) \
360 : : list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
361 : :
362 : : enum rwsem_wake_type {
363 : : RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
364 : : RWSEM_WAKE_READERS, /* Wake readers only */
365 : : RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
366 : : };
367 : :
368 : : enum writer_wait_state {
369 : : WRITER_NOT_FIRST, /* Writer is not first in wait list */
370 : : WRITER_FIRST, /* Writer is first in wait list */
371 : : WRITER_HANDOFF /* Writer is first & handoff needed */
372 : : };
373 : :
374 : : /*
375 : : * The typical HZ value is either 250 or 1000. So set the minimum waiting
376 : : * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
377 : : * queue before initiating the handoff protocol.
378 : : */
379 : : #define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
380 : :
381 : : /*
382 : : * Magic number to batch-wakeup waiting readers, even when writers are
383 : : * also present in the queue. This both limits the amount of work the
384 : : * waking thread must do and also prevents any potential counter overflow,
385 : : * however unlikely.
386 : : */
387 : : #define MAX_READERS_WAKEUP 0x100
388 : :
389 : : /*
390 : : * handle the lock release when processes blocked on it that can now run
391 : : * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
392 : : * have been set.
393 : : * - there must be someone on the queue
394 : : * - the wait_lock must be held by the caller
395 : : * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
396 : : * to actually wakeup the blocked task(s) and drop the reference count,
397 : : * preferably when the wait_lock is released
398 : : * - woken process blocks are discarded from the list after having task zeroed
399 : : * - writers are only marked woken if downgrading is false
400 : : */
401 : 3 : static void rwsem_mark_wake(struct rw_semaphore *sem,
402 : : enum rwsem_wake_type wake_type,
403 : : struct wake_q_head *wake_q)
404 : : {
405 : : struct rwsem_waiter *waiter, *tmp;
406 : : long oldcount, woken = 0, adjustment = 0;
407 : : struct list_head wlist;
408 : :
409 : : lockdep_assert_held(&sem->wait_lock);
410 : :
411 : : /*
412 : : * Take a peek at the queue head waiter such that we can determine
413 : : * the wakeup(s) to perform.
414 : : */
415 : 3 : waiter = rwsem_first_waiter(sem);
416 : :
417 : 3 : if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
418 : 3 : if (wake_type == RWSEM_WAKE_ANY) {
419 : : /*
420 : : * Mark writer at the front of the queue for wakeup.
421 : : * Until the task is actually later awoken later by
422 : : * the caller, other writers are able to steal it.
423 : : * Readers, on the other hand, will block as they
424 : : * will notice the queued writer.
425 : : */
426 : 3 : wake_q_add(wake_q, waiter->task);
427 : : lockevent_inc(rwsem_wake_writer);
428 : : }
429 : :
430 : 3 : return;
431 : : }
432 : :
433 : : /*
434 : : * No reader wakeup if there are too many of them already.
435 : : */
436 : 3 : if (unlikely(atomic_long_read(&sem->count) < 0))
437 : : return;
438 : :
439 : : /*
440 : : * Writers might steal the lock before we grant it to the next reader.
441 : : * We prefer to do the first reader grant before counting readers
442 : : * so we can bail out early if a writer stole the lock.
443 : : */
444 : 3 : if (wake_type != RWSEM_WAKE_READ_OWNED) {
445 : : struct task_struct *owner;
446 : :
447 : : adjustment = RWSEM_READER_BIAS;
448 : 3 : oldcount = atomic_long_fetch_add(adjustment, &sem->count);
449 : 3 : if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
450 : : /*
451 : : * When we've been waiting "too" long (for writers
452 : : * to give up the lock), request a HANDOFF to
453 : : * force the issue.
454 : : */
455 : 3 : if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
456 : 3 : time_after(jiffies, waiter->timeout)) {
457 : : adjustment -= RWSEM_FLAG_HANDOFF;
458 : : lockevent_inc(rwsem_rlock_handoff);
459 : : }
460 : :
461 : 3 : atomic_long_add(-adjustment, &sem->count);
462 : : return;
463 : : }
464 : : /*
465 : : * Set it to reader-owned to give spinners an early
466 : : * indication that readers now have the lock.
467 : : * The reader nonspinnable bit seen at slowpath entry of
468 : : * the reader is copied over.
469 : : */
470 : 3 : owner = waiter->task;
471 : 3 : if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
472 : 3 : owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
473 : : lockevent_inc(rwsem_opt_norspin);
474 : : }
475 : : __rwsem_set_reader_owned(sem, owner);
476 : : }
477 : :
478 : : /*
479 : : * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
480 : : * queue. We know that the woken will be at least 1 as we accounted
481 : : * for above. Note we increment the 'active part' of the count by the
482 : : * number of readers before waking any processes up.
483 : : *
484 : : * This is an adaptation of the phase-fair R/W locks where at the
485 : : * reader phase (first waiter is a reader), all readers are eligible
486 : : * to acquire the lock at the same time irrespective of their order
487 : : * in the queue. The writers acquire the lock according to their
488 : : * order in the queue.
489 : : *
490 : : * We have to do wakeup in 2 passes to prevent the possibility that
491 : : * the reader count may be decremented before it is incremented. It
492 : : * is because the to-be-woken waiter may not have slept yet. So it
493 : : * may see waiter->task got cleared, finish its critical section and
494 : : * do an unlock before the reader count increment.
495 : : *
496 : : * 1) Collect the read-waiters in a separate list, count them and
497 : : * fully increment the reader count in rwsem.
498 : : * 2) For each waiters in the new list, clear waiter->task and
499 : : * put them into wake_q to be woken up later.
500 : : */
501 : : INIT_LIST_HEAD(&wlist);
502 : 3 : list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
503 : 3 : if (waiter->type == RWSEM_WAITING_FOR_WRITE)
504 : 3 : continue;
505 : :
506 : 3 : woken++;
507 : : list_move_tail(&waiter->list, &wlist);
508 : :
509 : : /*
510 : : * Limit # of readers that can be woken up per wakeup call.
511 : : */
512 : 3 : if (woken >= MAX_READERS_WAKEUP)
513 : : break;
514 : : }
515 : :
516 : 3 : adjustment = woken * RWSEM_READER_BIAS - adjustment;
517 : : lockevent_cond_inc(rwsem_wake_reader, woken);
518 : 3 : if (list_empty(&sem->wait_list)) {
519 : : /* hit end of list above */
520 : 3 : adjustment -= RWSEM_FLAG_WAITERS;
521 : : }
522 : :
523 : : /*
524 : : * When we've woken a reader, we no longer need to force writers
525 : : * to give up the lock and we can clear HANDOFF.
526 : : */
527 : 3 : if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
528 : 3 : adjustment -= RWSEM_FLAG_HANDOFF;
529 : :
530 : 3 : if (adjustment)
531 : 3 : atomic_long_add(adjustment, &sem->count);
532 : :
533 : : /* 2nd pass */
534 : 3 : list_for_each_entry_safe(waiter, tmp, &wlist, list) {
535 : : struct task_struct *tsk;
536 : :
537 : 3 : tsk = waiter->task;
538 : : get_task_struct(tsk);
539 : :
540 : : /*
541 : : * Ensure calling get_task_struct() before setting the reader
542 : : * waiter to nil such that rwsem_down_read_slowpath() cannot
543 : : * race with do_exit() by always holding a reference count
544 : : * to the task to wakeup.
545 : : */
546 : 3 : smp_store_release(&waiter->task, NULL);
547 : : /*
548 : : * Ensure issuing the wakeup (either by us or someone else)
549 : : * after setting the reader waiter to nil.
550 : : */
551 : 3 : wake_q_add_safe(wake_q, tsk);
552 : : }
553 : : }
554 : :
555 : : /*
556 : : * This function must be called with the sem->wait_lock held to prevent
557 : : * race conditions between checking the rwsem wait list and setting the
558 : : * sem->count accordingly.
559 : : *
560 : : * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
561 : : * bit is set or the lock is acquired with handoff bit cleared.
562 : : */
563 : 3 : static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
564 : : enum writer_wait_state wstate)
565 : : {
566 : : long count, new;
567 : :
568 : : lockdep_assert_held(&sem->wait_lock);
569 : :
570 : 3 : count = atomic_long_read(&sem->count);
571 : : do {
572 : 3 : bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
573 : :
574 : 3 : if (has_handoff && wstate == WRITER_NOT_FIRST)
575 : : return false;
576 : :
577 : : new = count;
578 : :
579 : 3 : if (count & RWSEM_LOCK_MASK) {
580 : 3 : if (has_handoff || (wstate != WRITER_HANDOFF))
581 : : return false;
582 : :
583 : 3 : new |= RWSEM_FLAG_HANDOFF;
584 : : } else {
585 : 3 : new |= RWSEM_WRITER_LOCKED;
586 : 3 : new &= ~RWSEM_FLAG_HANDOFF;
587 : :
588 : 3 : if (list_is_singular(&sem->wait_list))
589 : 3 : new &= ~RWSEM_FLAG_WAITERS;
590 : : }
591 : 3 : } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
592 : :
593 : : /*
594 : : * We have either acquired the lock with handoff bit cleared or
595 : : * set the handoff bit.
596 : : */
597 : 3 : if (new & RWSEM_FLAG_HANDOFF)
598 : : return false;
599 : :
600 : : rwsem_set_owner(sem);
601 : 3 : return true;
602 : : }
603 : :
604 : : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
605 : : /*
606 : : * Try to acquire read lock before the reader is put on wait queue.
607 : : * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
608 : : * is ongoing.
609 : : */
610 : 3 : static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
611 : : {
612 : : long count = atomic_long_read(&sem->count);
613 : :
614 : 3 : if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
615 : : return false;
616 : :
617 : 3 : count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
618 : 3 : if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
619 : : rwsem_set_reader_owned(sem);
620 : : lockevent_inc(rwsem_opt_rlock);
621 : 3 : return true;
622 : : }
623 : :
624 : : /* Back out the change */
625 : : atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
626 : 3 : return false;
627 : : }
628 : :
629 : : /*
630 : : * Try to acquire write lock before the writer has been put on wait queue.
631 : : */
632 : 3 : static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
633 : : {
634 : 3 : long count = atomic_long_read(&sem->count);
635 : :
636 : 3 : while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
637 : 3 : if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
638 : 3 : count | RWSEM_WRITER_LOCKED)) {
639 : : rwsem_set_owner(sem);
640 : : lockevent_inc(rwsem_opt_wlock);
641 : 3 : return true;
642 : : }
643 : : }
644 : : return false;
645 : : }
646 : :
647 : : static inline bool owner_on_cpu(struct task_struct *owner)
648 : : {
649 : : /*
650 : : * As lock holder preemption issue, we both skip spinning if
651 : : * task is not on cpu or its cpu is preempted
652 : : */
653 : 3 : return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
654 : : }
655 : :
656 : 3 : static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
657 : : unsigned long nonspinnable)
658 : : {
659 : : struct task_struct *owner;
660 : : unsigned long flags;
661 : : bool ret = true;
662 : :
663 : : BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));
664 : :
665 : 3 : if (need_resched()) {
666 : : lockevent_inc(rwsem_opt_fail);
667 : : return false;
668 : : }
669 : :
670 : 3 : preempt_disable();
671 : : rcu_read_lock();
672 : : owner = rwsem_owner_flags(sem, &flags);
673 : : /*
674 : : * Don't check the read-owner as the entry may be stale.
675 : : */
676 : 3 : if ((flags & nonspinnable) ||
677 : 3 : (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
678 : : ret = false;
679 : : rcu_read_unlock();
680 : 3 : preempt_enable();
681 : :
682 : : lockevent_cond_inc(rwsem_opt_fail, !ret);
683 : 3 : return ret;
684 : : }
685 : :
686 : : /*
687 : : * The rwsem_spin_on_owner() function returns the folowing 4 values
688 : : * depending on the lock owner state.
689 : : * OWNER_NULL : owner is currently NULL
690 : : * OWNER_WRITER: when owner changes and is a writer
691 : : * OWNER_READER: when owner changes and the new owner may be a reader.
692 : : * OWNER_NONSPINNABLE:
693 : : * when optimistic spinning has to stop because either the
694 : : * owner stops running, is unknown, or its timeslice has
695 : : * been used up.
696 : : */
697 : : enum owner_state {
698 : : OWNER_NULL = 1 << 0,
699 : : OWNER_WRITER = 1 << 1,
700 : : OWNER_READER = 1 << 2,
701 : : OWNER_NONSPINNABLE = 1 << 3,
702 : : };
703 : : #define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER)
704 : :
705 : : static inline enum owner_state
706 : : rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)
707 : : {
708 : 3 : if (flags & nonspinnable)
709 : : return OWNER_NONSPINNABLE;
710 : :
711 : 3 : if (flags & RWSEM_READER_OWNED)
712 : : return OWNER_READER;
713 : :
714 : 3 : return owner ? OWNER_WRITER : OWNER_NULL;
715 : : }
716 : :
717 : : static noinline enum owner_state
718 : 3 : rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
719 : : {
720 : : struct task_struct *new, *owner;
721 : : unsigned long flags, new_flags;
722 : : enum owner_state state;
723 : :
724 : : owner = rwsem_owner_flags(sem, &flags);
725 : : state = rwsem_owner_state(owner, flags, nonspinnable);
726 : 3 : if (state != OWNER_WRITER)
727 : : return state;
728 : :
729 : : rcu_read_lock();
730 : : for (;;) {
731 : : /*
732 : : * When a waiting writer set the handoff flag, it may spin
733 : : * on the owner as well. Once that writer acquires the lock,
734 : : * we can spin on it. So we don't need to quit even when the
735 : : * handoff bit is set.
736 : : */
737 : : new = rwsem_owner_flags(sem, &new_flags);
738 : 3 : if ((new != owner) || (new_flags != flags)) {
739 : 3 : state = rwsem_owner_state(new, new_flags, nonspinnable);
740 : 3 : break;
741 : : }
742 : :
743 : : /*
744 : : * Ensure we emit the owner->on_cpu, dereference _after_
745 : : * checking sem->owner still matches owner, if that fails,
746 : : * owner might point to free()d memory, if it still matches,
747 : : * the rcu_read_lock() ensures the memory stays valid.
748 : : */
749 : 3 : barrier();
750 : :
751 : 3 : if (need_resched() || !owner_on_cpu(owner)) {
752 : : state = OWNER_NONSPINNABLE;
753 : : break;
754 : : }
755 : :
756 : 3 : cpu_relax();
757 : 3 : }
758 : : rcu_read_unlock();
759 : :
760 : 3 : return state;
761 : : }
762 : :
763 : : /*
764 : : * Calculate reader-owned rwsem spinning threshold for writer
765 : : *
766 : : * The more readers own the rwsem, the longer it will take for them to
767 : : * wind down and free the rwsem. So the empirical formula used to
768 : : * determine the actual spinning time limit here is:
769 : : *
770 : : * Spinning threshold = (10 + nr_readers/2)us
771 : : *
772 : : * The limit is capped to a maximum of 25us (30 readers). This is just
773 : : * a heuristic and is subjected to change in the future.
774 : : */
775 : 3 : static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
776 : : {
777 : : long count = atomic_long_read(&sem->count);
778 : 3 : int readers = count >> RWSEM_READER_SHIFT;
779 : : u64 delta;
780 : :
781 : 3 : if (readers > 30)
782 : : readers = 30;
783 : 3 : delta = (20 + readers) * NSEC_PER_USEC / 2;
784 : :
785 : 3 : return sched_clock() + delta;
786 : : }
787 : :
788 : 3 : static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
789 : : {
790 : : bool taken = false;
791 : : int prev_owner_state = OWNER_NULL;
792 : : int loop = 0;
793 : : u64 rspin_threshold = 0;
794 : : unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
795 : 3 : : RWSEM_RD_NONSPINNABLE;
796 : :
797 : 3 : preempt_disable();
798 : :
799 : : /* sem->wait_lock should not be held when doing optimistic spinning */
800 : 3 : if (!osq_lock(&sem->osq))
801 : : goto done;
802 : :
803 : : /*
804 : : * Optimistically spin on the owner field and attempt to acquire the
805 : : * lock whenever the owner changes. Spinning will be stopped when:
806 : : * 1) the owning writer isn't running; or
807 : : * 2) readers own the lock and spinning time has exceeded limit.
808 : : */
809 : : for (;;) {
810 : : enum owner_state owner_state;
811 : :
812 : 3 : owner_state = rwsem_spin_on_owner(sem, nonspinnable);
813 : 3 : if (!(owner_state & OWNER_SPINNABLE))
814 : : break;
815 : :
816 : : /*
817 : : * Try to acquire the lock
818 : : */
819 : 3 : taken = wlock ? rwsem_try_write_lock_unqueued(sem)
820 : 3 : : rwsem_try_read_lock_unqueued(sem);
821 : :
822 : 3 : if (taken)
823 : : break;
824 : :
825 : : /*
826 : : * Time-based reader-owned rwsem optimistic spinning
827 : : */
828 : 3 : if (wlock && (owner_state == OWNER_READER)) {
829 : : /*
830 : : * Re-initialize rspin_threshold every time when
831 : : * the owner state changes from non-reader to reader.
832 : : * This allows a writer to steal the lock in between
833 : : * 2 reader phases and have the threshold reset at
834 : : * the beginning of the 2nd reader phase.
835 : : */
836 : 3 : if (prev_owner_state != OWNER_READER) {
837 : 3 : if (rwsem_test_oflags(sem, nonspinnable))
838 : : break;
839 : 3 : rspin_threshold = rwsem_rspin_threshold(sem);
840 : : loop = 0;
841 : : }
842 : :
843 : : /*
844 : : * Check time threshold once every 16 iterations to
845 : : * avoid calling sched_clock() too frequently so
846 : : * as to reduce the average latency between the times
847 : : * when the lock becomes free and when the spinner
848 : : * is ready to do a trylock.
849 : : */
850 : 3 : else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
851 : 3 : rwsem_set_nonspinnable(sem);
852 : : lockevent_inc(rwsem_opt_nospin);
853 : 3 : break;
854 : : }
855 : : }
856 : :
857 : : /*
858 : : * An RT task cannot do optimistic spinning if it cannot
859 : : * be sure the lock holder is running or live-lock may
860 : : * happen if the current task and the lock holder happen
861 : : * to run in the same CPU. However, aborting optimistic
862 : : * spinning while a NULL owner is detected may miss some
863 : : * opportunity where spinning can continue without causing
864 : : * problem.
865 : : *
866 : : * There are 2 possible cases where an RT task may be able
867 : : * to continue spinning.
868 : : *
869 : : * 1) The lock owner is in the process of releasing the
870 : : * lock, sem->owner is cleared but the lock has not
871 : : * been released yet.
872 : : * 2) The lock was free and owner cleared, but another
873 : : * task just comes in and acquire the lock before
874 : : * we try to get it. The new owner may be a spinnable
875 : : * writer.
876 : : *
877 : : * To take advantage of two scenarios listed agove, the RT
878 : : * task is made to retry one more time to see if it can
879 : : * acquire the lock or continue spinning on the new owning
880 : : * writer. Of course, if the time lag is long enough or the
881 : : * new owner is not a writer or spinnable, the RT task will
882 : : * quit spinning.
883 : : *
884 : : * If the owner is a writer, the need_resched() check is
885 : : * done inside rwsem_spin_on_owner(). If the owner is not
886 : : * a writer, need_resched() check needs to be done here.
887 : : */
888 : 3 : if (owner_state != OWNER_WRITER) {
889 : 3 : if (need_resched())
890 : : break;
891 : 3 : if (rt_task(current) &&
892 : : (prev_owner_state != OWNER_WRITER))
893 : : break;
894 : : }
895 : 3 : prev_owner_state = owner_state;
896 : :
897 : : /*
898 : : * The cpu_relax() call is a compiler barrier which forces
899 : : * everything in this loop to be re-loaded. We don't need
900 : : * memory barriers as we'll eventually observe the right
901 : : * values at the cost of a few extra spins.
902 : : */
903 : 3 : cpu_relax();
904 : 3 : }
905 : 3 : osq_unlock(&sem->osq);
906 : : done:
907 : 3 : preempt_enable();
908 : : lockevent_cond_inc(rwsem_opt_fail, !taken);
909 : 3 : return taken;
910 : : }
911 : :
912 : : /*
913 : : * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
914 : : * only be called when the reader count reaches 0.
915 : : *
916 : : * This give writers better chance to acquire the rwsem first before
917 : : * readers when the rwsem was being held by readers for a relatively long
918 : : * period of time. Race can happen that an optimistic spinner may have
919 : : * just stolen the rwsem and set the owner, but just clearing the
920 : : * RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
921 : : */
922 : 3 : static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
923 : : {
924 : 3 : if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE))
925 : 3 : atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
926 : 3 : }
927 : :
928 : : /*
929 : : * This function is called when the reader fails to acquire the lock via
930 : : * optimistic spinning. In this case we will still attempt to do a trylock
931 : : * when comparing the rwsem state right now with the state when entering
932 : : * the slowpath indicates that the reader is still in a valid reader phase.
933 : : * This happens when the following conditions are true:
934 : : *
935 : : * 1) The lock is currently reader owned, and
936 : : * 2) The lock is previously not reader-owned or the last read owner changes.
937 : : *
938 : : * In the former case, we have transitioned from a writer phase to a
939 : : * reader-phase while spinning. In the latter case, it means the reader
940 : : * phase hasn't ended when we entered the optimistic spinning loop. In
941 : : * both cases, the reader is eligible to acquire the lock. This is the
942 : : * secondary path where a read lock is acquired optimistically.
943 : : *
944 : : * The reader non-spinnable bit wasn't set at time of entry or it will
945 : : * not be here at all.
946 : : */
947 : 3 : static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
948 : : unsigned long last_rowner)
949 : : {
950 : 3 : unsigned long owner = atomic_long_read(&sem->owner);
951 : :
952 : 3 : if (!(owner & RWSEM_READER_OWNED))
953 : : return false;
954 : :
955 : 3 : if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
956 : 3 : rwsem_try_read_lock_unqueued(sem)) {
957 : : lockevent_inc(rwsem_opt_rlock2);
958 : : lockevent_add(rwsem_opt_fail, -1);
959 : : return true;
960 : : }
961 : : return false;
962 : : }
963 : : #else
964 : : static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
965 : : unsigned long nonspinnable)
966 : : {
967 : : return false;
968 : : }
969 : :
970 : : static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
971 : : {
972 : : return false;
973 : : }
974 : :
975 : : static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }
976 : :
977 : : static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
978 : : unsigned long last_rowner)
979 : : {
980 : : return false;
981 : : }
982 : :
983 : : static inline int
984 : : rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
985 : : {
986 : : return 0;
987 : : }
988 : : #define OWNER_NULL 1
989 : : #endif
990 : :
991 : : /*
992 : : * Wait for the read lock to be granted
993 : : */
994 : : static struct rw_semaphore __sched *
995 : 3 : rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
996 : : {
997 : : long count, adjustment = -RWSEM_READER_BIAS;
998 : : struct rwsem_waiter waiter;
999 : 3 : DEFINE_WAKE_Q(wake_q);
1000 : : bool wake = false;
1001 : :
1002 : : /*
1003 : : * Save the current read-owner of rwsem, if available, and the
1004 : : * reader nonspinnable bit.
1005 : : */
1006 : 3 : waiter.last_rowner = atomic_long_read(&sem->owner);
1007 : 3 : if (!(waiter.last_rowner & RWSEM_READER_OWNED))
1008 : 3 : waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;
1009 : :
1010 : 3 : if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
1011 : : goto queue;
1012 : :
1013 : : /*
1014 : : * Undo read bias from down_read() and do optimistic spinning.
1015 : : */
1016 : 3 : atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
1017 : : adjustment = 0;
1018 : 3 : if (rwsem_optimistic_spin(sem, false)) {
1019 : : /* rwsem_optimistic_spin() implies ACQUIRE on success */
1020 : : /*
1021 : : * Wake up other readers in the wait list if the front
1022 : : * waiter is a reader.
1023 : : */
1024 : 3 : if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
1025 : 3 : raw_spin_lock_irq(&sem->wait_lock);
1026 : 3 : if (!list_empty(&sem->wait_list))
1027 : 3 : rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
1028 : : &wake_q);
1029 : 3 : raw_spin_unlock_irq(&sem->wait_lock);
1030 : 3 : wake_up_q(&wake_q);
1031 : : }
1032 : 3 : return sem;
1033 : 3 : } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
1034 : : /* rwsem_reader_phase_trylock() implies ACQUIRE on success */
1035 : : return sem;
1036 : : }
1037 : :
1038 : : queue:
1039 : 3 : waiter.task = current;
1040 : 3 : waiter.type = RWSEM_WAITING_FOR_READ;
1041 : 3 : waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1042 : :
1043 : 3 : raw_spin_lock_irq(&sem->wait_lock);
1044 : 3 : if (list_empty(&sem->wait_list)) {
1045 : : /*
1046 : : * In case the wait queue is empty and the lock isn't owned
1047 : : * by a writer or has the handoff bit set, this reader can
1048 : : * exit the slowpath and return immediately as its
1049 : : * RWSEM_READER_BIAS has already been set in the count.
1050 : : */
1051 : 3 : if (adjustment && !(atomic_long_read(&sem->count) &
1052 : : (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
1053 : : /* Provide lock ACQUIRE */
1054 : 3 : smp_acquire__after_ctrl_dep();
1055 : 3 : raw_spin_unlock_irq(&sem->wait_lock);
1056 : : rwsem_set_reader_owned(sem);
1057 : : lockevent_inc(rwsem_rlock_fast);
1058 : 3 : return sem;
1059 : : }
1060 : 3 : adjustment += RWSEM_FLAG_WAITERS;
1061 : : }
1062 : : list_add_tail(&waiter.list, &sem->wait_list);
1063 : :
1064 : : /* we're now waiting on the lock, but no longer actively locking */
1065 : 3 : if (adjustment)
1066 : 3 : count = atomic_long_add_return(adjustment, &sem->count);
1067 : : else
1068 : : count = atomic_long_read(&sem->count);
1069 : :
1070 : : /*
1071 : : * If there are no active locks, wake the front queued process(es).
1072 : : *
1073 : : * If there are no writers and we are first in the queue,
1074 : : * wake our own waiter to join the existing active readers !
1075 : : */
1076 : 3 : if (!(count & RWSEM_LOCK_MASK)) {
1077 : 3 : clear_wr_nonspinnable(sem);
1078 : : wake = true;
1079 : : }
1080 : 3 : if (wake || (!(count & RWSEM_WRITER_MASK) &&
1081 : 3 : (adjustment & RWSEM_FLAG_WAITERS)))
1082 : 3 : rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1083 : :
1084 : 3 : raw_spin_unlock_irq(&sem->wait_lock);
1085 : 3 : wake_up_q(&wake_q);
1086 : :
1087 : : /* wait to be given the lock */
1088 : : for (;;) {
1089 : 3 : set_current_state(state);
1090 : 3 : if (!smp_load_acquire(&waiter.task)) {
1091 : : /* Matches rwsem_mark_wake()'s smp_store_release(). */
1092 : : break;
1093 : : }
1094 : 3 : if (signal_pending_state(state, current)) {
1095 : 0 : raw_spin_lock_irq(&sem->wait_lock);
1096 : 0 : if (waiter.task)
1097 : : goto out_nolock;
1098 : 0 : raw_spin_unlock_irq(&sem->wait_lock);
1099 : : /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
1100 : 0 : break;
1101 : : }
1102 : 3 : schedule();
1103 : : lockevent_inc(rwsem_sleep_reader);
1104 : 3 : }
1105 : :
1106 : 3 : __set_current_state(TASK_RUNNING);
1107 : : lockevent_inc(rwsem_rlock);
1108 : 3 : return sem;
1109 : :
1110 : : out_nolock:
1111 : : list_del(&waiter.list);
1112 : 0 : if (list_empty(&sem->wait_list)) {
1113 : 0 : atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
1114 : : &sem->count);
1115 : : }
1116 : 0 : raw_spin_unlock_irq(&sem->wait_lock);
1117 : 0 : __set_current_state(TASK_RUNNING);
1118 : : lockevent_inc(rwsem_rlock_fail);
1119 : 0 : return ERR_PTR(-EINTR);
1120 : : }
1121 : :
1122 : : /*
1123 : : * This function is called by the a write lock owner. So the owner value
1124 : : * won't get changed by others.
1125 : : */
1126 : 3 : static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
1127 : : bool disable)
1128 : : {
1129 : 3 : if (unlikely(disable)) {
1130 : 3 : atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
1131 : : lockevent_inc(rwsem_opt_norspin);
1132 : : }
1133 : 3 : }
1134 : :
1135 : : /*
1136 : : * Wait until we successfully acquire the write lock
1137 : : */
1138 : : static struct rw_semaphore *
1139 : 3 : rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
1140 : : {
1141 : : long count;
1142 : : bool disable_rspin;
1143 : : enum writer_wait_state wstate;
1144 : : struct rwsem_waiter waiter;
1145 : : struct rw_semaphore *ret = sem;
1146 : 3 : DEFINE_WAKE_Q(wake_q);
1147 : :
1148 : : /* do optimistic spinning and steal lock if possible */
1149 : 3 : if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
1150 : 3 : rwsem_optimistic_spin(sem, true)) {
1151 : : /* rwsem_optimistic_spin() implies ACQUIRE on success */
1152 : : return sem;
1153 : : }
1154 : :
1155 : : /*
1156 : : * Disable reader optimistic spinning for this rwsem after
1157 : : * acquiring the write lock when the setting of the nonspinnable
1158 : : * bits are observed.
1159 : : */
1160 : 3 : disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;
1161 : :
1162 : : /*
1163 : : * Optimistic spinning failed, proceed to the slowpath
1164 : : * and block until we can acquire the sem.
1165 : : */
1166 : 3 : waiter.task = current;
1167 : 3 : waiter.type = RWSEM_WAITING_FOR_WRITE;
1168 : 3 : waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
1169 : :
1170 : 3 : raw_spin_lock_irq(&sem->wait_lock);
1171 : :
1172 : : /* account for this before adding a new element to the list */
1173 : 3 : wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
1174 : :
1175 : : list_add_tail(&waiter.list, &sem->wait_list);
1176 : :
1177 : : /* we're now waiting on the lock */
1178 : 3 : if (wstate == WRITER_NOT_FIRST) {
1179 : : count = atomic_long_read(&sem->count);
1180 : :
1181 : : /*
1182 : : * If there were already threads queued before us and:
1183 : : * 1) there are no no active locks, wake the front
1184 : : * queued process(es) as the handoff bit might be set.
1185 : : * 2) there are no active writers and some readers, the lock
1186 : : * must be read owned; so we try to wake any read lock
1187 : : * waiters that were queued ahead of us.
1188 : : */
1189 : 3 : if (count & RWSEM_WRITER_MASK)
1190 : : goto wait;
1191 : :
1192 : 3 : rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
1193 : : ? RWSEM_WAKE_READERS
1194 : : : RWSEM_WAKE_ANY, &wake_q);
1195 : :
1196 : 3 : if (!wake_q_empty(&wake_q)) {
1197 : : /*
1198 : : * We want to minimize wait_lock hold time especially
1199 : : * when a large number of readers are to be woken up.
1200 : : */
1201 : 3 : raw_spin_unlock_irq(&sem->wait_lock);
1202 : 3 : wake_up_q(&wake_q);
1203 : : wake_q_init(&wake_q); /* Used again, reinit */
1204 : 3 : raw_spin_lock_irq(&sem->wait_lock);
1205 : : }
1206 : : } else {
1207 : 3 : atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
1208 : : }
1209 : :
1210 : : wait:
1211 : : /* wait until we successfully acquire the lock */
1212 : 3 : set_current_state(state);
1213 : : for (;;) {
1214 : 3 : if (rwsem_try_write_lock(sem, wstate)) {
1215 : : /* rwsem_try_write_lock() implies ACQUIRE on success */
1216 : : break;
1217 : : }
1218 : :
1219 : 3 : raw_spin_unlock_irq(&sem->wait_lock);
1220 : :
1221 : : /*
1222 : : * After setting the handoff bit and failing to acquire
1223 : : * the lock, attempt to spin on owner to accelerate lock
1224 : : * transfer. If the previous owner is a on-cpu writer and it
1225 : : * has just released the lock, OWNER_NULL will be returned.
1226 : : * In this case, we attempt to acquire the lock again
1227 : : * without sleeping.
1228 : : */
1229 : 3 : if (wstate == WRITER_HANDOFF &&
1230 : 3 : rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
1231 : : goto trylock_again;
1232 : :
1233 : : /* Block until there are no active lockers. */
1234 : : for (;;) {
1235 : 3 : if (signal_pending_state(state, current))
1236 : : goto out_nolock;
1237 : :
1238 : 3 : schedule();
1239 : : lockevent_inc(rwsem_sleep_writer);
1240 : 3 : set_current_state(state);
1241 : : /*
1242 : : * If HANDOFF bit is set, unconditionally do
1243 : : * a trylock.
1244 : : */
1245 : 3 : if (wstate == WRITER_HANDOFF)
1246 : : break;
1247 : :
1248 : 3 : if ((wstate == WRITER_NOT_FIRST) &&
1249 : 3 : (rwsem_first_waiter(sem) == &waiter))
1250 : : wstate = WRITER_FIRST;
1251 : :
1252 : : count = atomic_long_read(&sem->count);
1253 : 3 : if (!(count & RWSEM_LOCK_MASK))
1254 : : break;
1255 : :
1256 : : /*
1257 : : * The setting of the handoff bit is deferred
1258 : : * until rwsem_try_write_lock() is called.
1259 : : */
1260 : 3 : if ((wstate == WRITER_FIRST) && (rt_task(current) ||
1261 : 3 : time_after(jiffies, waiter.timeout))) {
1262 : : wstate = WRITER_HANDOFF;
1263 : : lockevent_inc(rwsem_wlock_handoff);
1264 : : break;
1265 : : }
1266 : : }
1267 : : trylock_again:
1268 : 3 : raw_spin_lock_irq(&sem->wait_lock);
1269 : 3 : }
1270 : 3 : __set_current_state(TASK_RUNNING);
1271 : : list_del(&waiter.list);
1272 : 3 : rwsem_disable_reader_optspin(sem, disable_rspin);
1273 : 3 : raw_spin_unlock_irq(&sem->wait_lock);
1274 : : lockevent_inc(rwsem_wlock);
1275 : :
1276 : 3 : return ret;
1277 : :
1278 : : out_nolock:
1279 : 0 : __set_current_state(TASK_RUNNING);
1280 : 0 : raw_spin_lock_irq(&sem->wait_lock);
1281 : : list_del(&waiter.list);
1282 : :
1283 : 0 : if (unlikely(wstate == WRITER_HANDOFF))
1284 : 0 : atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count);
1285 : :
1286 : 0 : if (list_empty(&sem->wait_list))
1287 : 0 : atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
1288 : : else
1289 : 0 : rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1290 : 0 : raw_spin_unlock_irq(&sem->wait_lock);
1291 : 0 : wake_up_q(&wake_q);
1292 : : lockevent_inc(rwsem_wlock_fail);
1293 : :
1294 : 0 : return ERR_PTR(-EINTR);
1295 : : }
1296 : :
1297 : : /*
1298 : : * handle waking up a waiter on the semaphore
1299 : : * - up_read/up_write has decremented the active part of count if we come here
1300 : : */
1301 : 3 : static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
1302 : : {
1303 : : unsigned long flags;
1304 : 3 : DEFINE_WAKE_Q(wake_q);
1305 : :
1306 : 3 : raw_spin_lock_irqsave(&sem->wait_lock, flags);
1307 : :
1308 : 3 : if (!list_empty(&sem->wait_list))
1309 : 3 : rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
1310 : :
1311 : 3 : raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1312 : 3 : wake_up_q(&wake_q);
1313 : :
1314 : 3 : return sem;
1315 : : }
1316 : :
1317 : : /*
1318 : : * downgrade a write lock into a read lock
1319 : : * - caller incremented waiting part of count and discovered it still negative
1320 : : * - just wake up any readers at the front of the queue
1321 : : */
1322 : 3 : static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
1323 : : {
1324 : : unsigned long flags;
1325 : 3 : DEFINE_WAKE_Q(wake_q);
1326 : :
1327 : 3 : raw_spin_lock_irqsave(&sem->wait_lock, flags);
1328 : :
1329 : 3 : if (!list_empty(&sem->wait_list))
1330 : 3 : rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
1331 : :
1332 : 3 : raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
1333 : 3 : wake_up_q(&wake_q);
1334 : :
1335 : 3 : return sem;
1336 : : }
1337 : :
1338 : : /*
1339 : : * lock for reading
1340 : : */
1341 : 3 : inline void __down_read(struct rw_semaphore *sem)
1342 : : {
1343 : 3 : if (!rwsem_read_trylock(sem)) {
1344 : 3 : rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
1345 : : DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1346 : : } else {
1347 : : rwsem_set_reader_owned(sem);
1348 : : }
1349 : 3 : }
1350 : :
1351 : 3 : static inline int __down_read_killable(struct rw_semaphore *sem)
1352 : : {
1353 : 3 : if (!rwsem_read_trylock(sem)) {
1354 : 3 : if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
1355 : : return -EINTR;
1356 : : DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1357 : : } else {
1358 : : rwsem_set_reader_owned(sem);
1359 : : }
1360 : : return 0;
1361 : : }
1362 : :
1363 : 3 : static inline int __down_read_trylock(struct rw_semaphore *sem)
1364 : : {
1365 : : long tmp;
1366 : :
1367 : : DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1368 : :
1369 : : /*
1370 : : * Optimize for the case when the rwsem is not locked at all.
1371 : : */
1372 : 3 : tmp = RWSEM_UNLOCKED_VALUE;
1373 : : do {
1374 : 3 : if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1375 : 3 : tmp + RWSEM_READER_BIAS)) {
1376 : : rwsem_set_reader_owned(sem);
1377 : 3 : return 1;
1378 : : }
1379 : 3 : } while (!(tmp & RWSEM_READ_FAILED_MASK));
1380 : : return 0;
1381 : : }
1382 : :
1383 : : /*
1384 : : * lock for writing
1385 : : */
1386 : 3 : static inline void __down_write(struct rw_semaphore *sem)
1387 : : {
1388 : 3 : long tmp = RWSEM_UNLOCKED_VALUE;
1389 : :
1390 : 3 : if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1391 : : RWSEM_WRITER_LOCKED)))
1392 : 3 : rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
1393 : : else
1394 : : rwsem_set_owner(sem);
1395 : 3 : }
1396 : :
1397 : 3 : static inline int __down_write_killable(struct rw_semaphore *sem)
1398 : : {
1399 : 3 : long tmp = RWSEM_UNLOCKED_VALUE;
1400 : :
1401 : 3 : if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1402 : : RWSEM_WRITER_LOCKED))) {
1403 : 3 : if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
1404 : : return -EINTR;
1405 : : } else {
1406 : : rwsem_set_owner(sem);
1407 : : }
1408 : : return 0;
1409 : : }
1410 : :
1411 : 3 : static inline int __down_write_trylock(struct rw_semaphore *sem)
1412 : : {
1413 : : long tmp;
1414 : :
1415 : : DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1416 : :
1417 : 3 : tmp = RWSEM_UNLOCKED_VALUE;
1418 : 3 : if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
1419 : : RWSEM_WRITER_LOCKED)) {
1420 : : rwsem_set_owner(sem);
1421 : 3 : return true;
1422 : : }
1423 : : return false;
1424 : : }
1425 : :
1426 : : /*
1427 : : * unlock after reading
1428 : : */
1429 : 3 : inline void __up_read(struct rw_semaphore *sem)
1430 : : {
1431 : : long tmp;
1432 : :
1433 : : DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1434 : : DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1435 : :
1436 : : rwsem_clear_reader_owned(sem);
1437 : 3 : tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
1438 : : DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
1439 : 3 : if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
1440 : : RWSEM_FLAG_WAITERS)) {
1441 : 3 : clear_wr_nonspinnable(sem);
1442 : 3 : rwsem_wake(sem, tmp);
1443 : : }
1444 : 3 : }
1445 : :
1446 : : /*
1447 : : * unlock after writing
1448 : : */
1449 : 3 : static inline void __up_write(struct rw_semaphore *sem)
1450 : : {
1451 : : long tmp;
1452 : :
1453 : : DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
1454 : : /*
1455 : : * sem->owner may differ from current if the ownership is transferred
1456 : : * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
1457 : : */
1458 : : DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
1459 : : !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
1460 : :
1461 : : rwsem_clear_owner(sem);
1462 : 3 : tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
1463 : 3 : if (unlikely(tmp & RWSEM_FLAG_WAITERS))
1464 : 3 : rwsem_wake(sem, tmp);
1465 : 3 : }
1466 : :
1467 : : /*
1468 : : * downgrade write lock to read lock
1469 : : */
1470 : 3 : static inline void __downgrade_write(struct rw_semaphore *sem)
1471 : : {
1472 : : long tmp;
1473 : :
1474 : : /*
1475 : : * When downgrading from exclusive to shared ownership,
1476 : : * anything inside the write-locked region cannot leak
1477 : : * into the read side. In contrast, anything in the
1478 : : * read-locked region is ok to be re-ordered into the
1479 : : * write side. As such, rely on RELEASE semantics.
1480 : : */
1481 : : DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
1482 : 3 : tmp = atomic_long_fetch_add_release(
1483 : : -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
1484 : : rwsem_set_reader_owned(sem);
1485 : 3 : if (tmp & RWSEM_FLAG_WAITERS)
1486 : 3 : rwsem_downgrade_wake(sem);
1487 : 3 : }
1488 : :
1489 : : /*
1490 : : * lock for reading
1491 : : */
1492 : 3 : void __sched down_read(struct rw_semaphore *sem)
1493 : : {
1494 : 3 : might_sleep();
1495 : : rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1496 : :
1497 : 3 : LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1498 : 3 : }
1499 : : EXPORT_SYMBOL(down_read);
1500 : :
1501 : 3 : int __sched down_read_killable(struct rw_semaphore *sem)
1502 : : {
1503 : 3 : might_sleep();
1504 : : rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
1505 : :
1506 : 3 : if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
1507 : : rwsem_release(&sem->dep_map, 1, _RET_IP_);
1508 : : return -EINTR;
1509 : : }
1510 : :
1511 : 3 : return 0;
1512 : : }
1513 : : EXPORT_SYMBOL(down_read_killable);
1514 : :
1515 : : /*
1516 : : * trylock for reading -- returns 1 if successful, 0 if contention
1517 : : */
1518 : 3 : int down_read_trylock(struct rw_semaphore *sem)
1519 : : {
1520 : 3 : int ret = __down_read_trylock(sem);
1521 : :
1522 : : if (ret == 1)
1523 : : rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
1524 : 3 : return ret;
1525 : : }
1526 : : EXPORT_SYMBOL(down_read_trylock);
1527 : :
1528 : : /*
1529 : : * lock for writing
1530 : : */
1531 : 3 : void __sched down_write(struct rw_semaphore *sem)
1532 : : {
1533 : 3 : might_sleep();
1534 : : rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1535 : 3 : LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1536 : 3 : }
1537 : : EXPORT_SYMBOL(down_write);
1538 : :
1539 : : /*
1540 : : * lock for writing
1541 : : */
1542 : 3 : int __sched down_write_killable(struct rw_semaphore *sem)
1543 : : {
1544 : 3 : might_sleep();
1545 : : rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
1546 : :
1547 : 3 : if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1548 : : __down_write_killable)) {
1549 : : rwsem_release(&sem->dep_map, 1, _RET_IP_);
1550 : : return -EINTR;
1551 : : }
1552 : :
1553 : 3 : return 0;
1554 : : }
1555 : : EXPORT_SYMBOL(down_write_killable);
1556 : :
1557 : : /*
1558 : : * trylock for writing -- returns 1 if successful, 0 if contention
1559 : : */
1560 : 3 : int down_write_trylock(struct rw_semaphore *sem)
1561 : : {
1562 : 3 : int ret = __down_write_trylock(sem);
1563 : :
1564 : : if (ret == 1)
1565 : : rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
1566 : :
1567 : 3 : return ret;
1568 : : }
1569 : : EXPORT_SYMBOL(down_write_trylock);
1570 : :
1571 : : /*
1572 : : * release a read lock
1573 : : */
1574 : 3 : void up_read(struct rw_semaphore *sem)
1575 : : {
1576 : : rwsem_release(&sem->dep_map, 1, _RET_IP_);
1577 : 3 : __up_read(sem);
1578 : 3 : }
1579 : : EXPORT_SYMBOL(up_read);
1580 : :
1581 : : /*
1582 : : * release a write lock
1583 : : */
1584 : 3 : void up_write(struct rw_semaphore *sem)
1585 : : {
1586 : : rwsem_release(&sem->dep_map, 1, _RET_IP_);
1587 : 3 : __up_write(sem);
1588 : 3 : }
1589 : : EXPORT_SYMBOL(up_write);
1590 : :
1591 : : /*
1592 : : * downgrade write lock to read lock
1593 : : */
1594 : 3 : void downgrade_write(struct rw_semaphore *sem)
1595 : : {
1596 : : lock_downgrade(&sem->dep_map, _RET_IP_);
1597 : 3 : __downgrade_write(sem);
1598 : 3 : }
1599 : : EXPORT_SYMBOL(downgrade_write);
1600 : :
1601 : : #ifdef CONFIG_DEBUG_LOCK_ALLOC
1602 : :
1603 : : void down_read_nested(struct rw_semaphore *sem, int subclass)
1604 : : {
1605 : : might_sleep();
1606 : : rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
1607 : : LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
1608 : : }
1609 : : EXPORT_SYMBOL(down_read_nested);
1610 : :
1611 : : void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
1612 : : {
1613 : : might_sleep();
1614 : : rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
1615 : : LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1616 : : }
1617 : : EXPORT_SYMBOL(_down_write_nest_lock);
1618 : :
1619 : : void down_read_non_owner(struct rw_semaphore *sem)
1620 : : {
1621 : : might_sleep();
1622 : : __down_read(sem);
1623 : : __rwsem_set_reader_owned(sem, NULL);
1624 : : }
1625 : : EXPORT_SYMBOL(down_read_non_owner);
1626 : :
1627 : : void down_write_nested(struct rw_semaphore *sem, int subclass)
1628 : : {
1629 : : might_sleep();
1630 : : rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1631 : : LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
1632 : : }
1633 : : EXPORT_SYMBOL(down_write_nested);
1634 : :
1635 : : int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
1636 : : {
1637 : : might_sleep();
1638 : : rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
1639 : :
1640 : : if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
1641 : : __down_write_killable)) {
1642 : : rwsem_release(&sem->dep_map, 1, _RET_IP_);
1643 : : return -EINTR;
1644 : : }
1645 : :
1646 : : return 0;
1647 : : }
1648 : : EXPORT_SYMBOL(down_write_killable_nested);
1649 : :
1650 : : void up_read_non_owner(struct rw_semaphore *sem)
1651 : : {
1652 : : DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
1653 : : __up_read(sem);
1654 : : }
1655 : : EXPORT_SYMBOL(up_read_non_owner);
1656 : :
1657 : : #endif
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