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