1 /* rwsem.c: R/W semaphores: contention handling functions
3 * Written by David Howells (dhowells@redhat.com).
4 * Derived from arch/i386/kernel/semaphore.c
6 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
7 * and Michel Lespinasse <walken@google.com>
9 * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
10 * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
12 #include <linux/rwsem.h>
13 #include <linux/init.h>
14 #include <linux/export.h>
15 #include <linux/sched/signal.h>
16 #include <linux/sched/rt.h>
17 #include <linux/sched/wake_q.h>
18 #include <linux/osq_lock.h>
23 * Guide to the rw_semaphore's count field for common values.
24 * (32-bit case illustrated, similar for 64-bit)
26 * 0x0000000X (1) X readers active or attempting lock, no writer waiting
27 * X = #active_readers + #readers attempting to lock
30 * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
31 * attempting to read lock or write lock.
33 * 0xffff000X (1) X readers active or attempting lock, with waiters for lock
34 * X = #active readers + # readers attempting lock
35 * (X*ACTIVE_BIAS + WAITING_BIAS)
36 * (2) 1 writer attempting lock, no waiters for lock
37 * X-1 = #active readers + #readers attempting lock
38 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
39 * (3) 1 writer active, no waiters for lock
40 * X-1 = #active readers + #readers attempting lock
41 * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
43 * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
44 * (WAITING_BIAS + ACTIVE_BIAS)
45 * (2) 1 writer active or attempting lock, no waiters for lock
48 * 0xffff0000 (1) There are writers or readers queued but none active
49 * or in the process of attempting lock.
51 * Note: writer can attempt to steal lock for this count by adding
52 * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
54 * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
55 * (ACTIVE_WRITE_BIAS + WAITING_BIAS)
57 * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
58 * the count becomes more than 0 for successful lock acquisition,
59 * i.e. the case where there are only readers or nobody has lock.
60 * (1st and 2nd case above).
62 * Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
63 * checking the count becomes ACTIVE_WRITE_BIAS for successful lock
64 * acquisition (i.e. nobody else has lock or attempts lock). If
65 * unsuccessful, in rwsem_down_write_failed, we'll check to see if there
66 * are only waiters but none active (5th case above), and attempt to
72 * Initialize an rwsem:
74 void __init_rwsem(struct rw_semaphore
*sem
, const char *name
,
75 struct lock_class_key
*key
)
77 #ifdef CONFIG_DEBUG_LOCK_ALLOC
79 * Make sure we are not reinitializing a held semaphore:
81 debug_check_no_locks_freed((void *)sem
, sizeof(*sem
));
82 lockdep_init_map(&sem
->dep_map
, name
, key
, 0);
84 atomic_long_set(&sem
->count
, RWSEM_UNLOCKED_VALUE
);
85 raw_spin_lock_init(&sem
->wait_lock
);
86 INIT_LIST_HEAD(&sem
->wait_list
);
87 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
89 osq_lock_init(&sem
->osq
);
93 EXPORT_SYMBOL(__init_rwsem
);
95 enum rwsem_waiter_type
{
96 RWSEM_WAITING_FOR_WRITE
,
97 RWSEM_WAITING_FOR_READ
100 struct rwsem_waiter
{
101 struct list_head list
;
102 struct task_struct
*task
;
103 enum rwsem_waiter_type type
;
106 enum rwsem_wake_type
{
107 RWSEM_WAKE_ANY
, /* Wake whatever's at head of wait list */
108 RWSEM_WAKE_READERS
, /* Wake readers only */
109 RWSEM_WAKE_READ_OWNED
/* Waker thread holds the read lock */
113 * handle the lock release when processes blocked on it that can now run
114 * - if we come here from up_xxxx(), then:
115 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
116 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
117 * - there must be someone on the queue
118 * - the wait_lock must be held by the caller
119 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
120 * to actually wakeup the blocked task(s) and drop the reference count,
121 * preferably when the wait_lock is released
122 * - woken process blocks are discarded from the list after having task zeroed
123 * - writers are only marked woken if downgrading is false
125 static void __rwsem_mark_wake(struct rw_semaphore
*sem
,
126 enum rwsem_wake_type wake_type
,
127 struct wake_q_head
*wake_q
)
129 struct rwsem_waiter
*waiter
, *tmp
;
130 long oldcount
, woken
= 0, adjustment
= 0;
133 * Take a peek at the queue head waiter such that we can determine
134 * the wakeup(s) to perform.
136 waiter
= list_first_entry(&sem
->wait_list
, struct rwsem_waiter
, list
);
138 if (waiter
->type
== RWSEM_WAITING_FOR_WRITE
) {
139 if (wake_type
== RWSEM_WAKE_ANY
) {
141 * Mark writer at the front of the queue for wakeup.
142 * Until the task is actually later awoken later by
143 * the caller, other writers are able to steal it.
144 * Readers, on the other hand, will block as they
145 * will notice the queued writer.
147 wake_q_add(wake_q
, waiter
->task
);
154 * Writers might steal the lock before we grant it to the next reader.
155 * We prefer to do the first reader grant before counting readers
156 * so we can bail out early if a writer stole the lock.
158 if (wake_type
!= RWSEM_WAKE_READ_OWNED
) {
159 adjustment
= RWSEM_ACTIVE_READ_BIAS
;
161 oldcount
= atomic_long_fetch_add(adjustment
, &sem
->count
);
162 if (unlikely(oldcount
< RWSEM_WAITING_BIAS
)) {
164 * If the count is still less than RWSEM_WAITING_BIAS
165 * after removing the adjustment, it is assumed that
166 * a writer has stolen the lock. We have to undo our
169 if (atomic_long_add_return(-adjustment
, &sem
->count
) <
173 /* Last active locker left. Retry waking readers. */
174 goto try_reader_grant
;
177 * It is not really necessary to set it to reader-owned here,
178 * but it gives the spinners an early indication that the
179 * readers now have the lock.
181 rwsem_set_reader_owned(sem
);
185 * Grant an infinite number of read locks to the readers at the front
186 * of the queue. We know that woken will be at least 1 as we accounted
187 * for above. Note we increment the 'active part' of the count by the
188 * number of readers before waking any processes up.
190 list_for_each_entry_safe(waiter
, tmp
, &sem
->wait_list
, list
) {
191 struct task_struct
*tsk
;
193 if (waiter
->type
== RWSEM_WAITING_FOR_WRITE
)
199 wake_q_add(wake_q
, tsk
);
200 list_del(&waiter
->list
);
202 * Ensure that the last operation is setting the reader
203 * waiter to nil such that rwsem_down_read_failed() cannot
204 * race with do_exit() by always holding a reference count
205 * to the task to wakeup.
207 smp_store_release(&waiter
->task
, NULL
);
210 adjustment
= woken
* RWSEM_ACTIVE_READ_BIAS
- adjustment
;
211 if (list_empty(&sem
->wait_list
)) {
212 /* hit end of list above */
213 adjustment
-= RWSEM_WAITING_BIAS
;
217 atomic_long_add(adjustment
, &sem
->count
);
221 * Wait for the read lock to be granted
224 struct rw_semaphore __sched
*rwsem_down_read_failed(struct rw_semaphore
*sem
)
226 long count
, adjustment
= -RWSEM_ACTIVE_READ_BIAS
;
227 struct rwsem_waiter waiter
;
228 DEFINE_WAKE_Q(wake_q
);
230 waiter
.task
= current
;
231 waiter
.type
= RWSEM_WAITING_FOR_READ
;
233 raw_spin_lock_irq(&sem
->wait_lock
);
234 if (list_empty(&sem
->wait_list
))
235 adjustment
+= RWSEM_WAITING_BIAS
;
236 list_add_tail(&waiter
.list
, &sem
->wait_list
);
238 /* we're now waiting on the lock, but no longer actively locking */
239 count
= atomic_long_add_return(adjustment
, &sem
->count
);
242 * If there are no active locks, wake the front queued process(es).
244 * If there are no writers and we are first in the queue,
245 * wake our own waiter to join the existing active readers !
247 if (count
== RWSEM_WAITING_BIAS
||
248 (count
> RWSEM_WAITING_BIAS
&&
249 adjustment
!= -RWSEM_ACTIVE_READ_BIAS
))
250 __rwsem_mark_wake(sem
, RWSEM_WAKE_ANY
, &wake_q
);
252 raw_spin_unlock_irq(&sem
->wait_lock
);
255 /* wait to be given the lock */
257 set_current_state(TASK_UNINTERRUPTIBLE
);
263 __set_current_state(TASK_RUNNING
);
266 EXPORT_SYMBOL(rwsem_down_read_failed
);
269 * This function must be called with the sem->wait_lock held to prevent
270 * race conditions between checking the rwsem wait list and setting the
271 * sem->count accordingly.
273 static inline bool rwsem_try_write_lock(long count
, struct rw_semaphore
*sem
)
276 * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
278 if (count
!= RWSEM_WAITING_BIAS
)
282 * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
283 * are other tasks on the wait list, we need to add on WAITING_BIAS.
285 count
= list_is_singular(&sem
->wait_list
) ?
286 RWSEM_ACTIVE_WRITE_BIAS
:
287 RWSEM_ACTIVE_WRITE_BIAS
+ RWSEM_WAITING_BIAS
;
289 if (atomic_long_cmpxchg_acquire(&sem
->count
, RWSEM_WAITING_BIAS
, count
)
290 == RWSEM_WAITING_BIAS
) {
291 rwsem_set_owner(sem
);
298 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
300 * Try to acquire write lock before the writer has been put on wait queue.
302 static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore
*sem
)
304 long old
, count
= atomic_long_read(&sem
->count
);
307 if (!(count
== 0 || count
== RWSEM_WAITING_BIAS
))
310 old
= atomic_long_cmpxchg_acquire(&sem
->count
, count
,
311 count
+ RWSEM_ACTIVE_WRITE_BIAS
);
313 rwsem_set_owner(sem
);
321 static inline bool rwsem_can_spin_on_owner(struct rw_semaphore
*sem
)
323 struct task_struct
*owner
;
330 owner
= READ_ONCE(sem
->owner
);
331 if (!rwsem_owner_is_writer(owner
)) {
333 * Don't spin if the rwsem is readers owned.
335 ret
= !rwsem_owner_is_reader(owner
);
340 * As lock holder preemption issue, we both skip spinning if task is not
341 * on cpu or its cpu is preempted
343 ret
= owner
->on_cpu
&& !vcpu_is_preempted(task_cpu(owner
));
350 * Return true only if we can still spin on the owner field of the rwsem.
352 static noinline
bool rwsem_spin_on_owner(struct rw_semaphore
*sem
)
354 struct task_struct
*owner
= READ_ONCE(sem
->owner
);
356 if (!rwsem_owner_is_writer(owner
))
360 while (sem
->owner
== owner
) {
362 * Ensure we emit the owner->on_cpu, dereference _after_
363 * checking sem->owner still matches owner, if that fails,
364 * owner might point to free()d memory, if it still matches,
365 * the rcu_read_lock() ensures the memory stays valid.
370 * abort spinning when need_resched or owner is not running or
371 * owner's cpu is preempted.
373 if (!owner
->on_cpu
|| need_resched() ||
374 vcpu_is_preempted(task_cpu(owner
))) {
384 * If there is a new owner or the owner is not set, we continue
387 return !rwsem_owner_is_reader(READ_ONCE(sem
->owner
));
390 static bool rwsem_optimistic_spin(struct rw_semaphore
*sem
)
396 /* sem->wait_lock should not be held when doing optimistic spinning */
397 if (!rwsem_can_spin_on_owner(sem
))
400 if (!osq_lock(&sem
->osq
))
404 * Optimistically spin on the owner field and attempt to acquire the
405 * lock whenever the owner changes. Spinning will be stopped when:
406 * 1) the owning writer isn't running; or
407 * 2) readers own the lock as we can't determine if they are
408 * actively running or not.
410 while (rwsem_spin_on_owner(sem
)) {
412 * Try to acquire the lock
414 if (rwsem_try_write_lock_unqueued(sem
)) {
420 * When there's no owner, we might have preempted between the
421 * owner acquiring the lock and setting the owner field. If
422 * we're an RT task that will live-lock because we won't let
423 * the owner complete.
425 if (!sem
->owner
&& (need_resched() || rt_task(current
)))
429 * The cpu_relax() call is a compiler barrier which forces
430 * everything in this loop to be re-loaded. We don't need
431 * memory barriers as we'll eventually observe the right
432 * values at the cost of a few extra spins.
436 osq_unlock(&sem
->osq
);
443 * Return true if the rwsem has active spinner
445 static inline bool rwsem_has_spinner(struct rw_semaphore
*sem
)
447 return osq_is_locked(&sem
->osq
);
451 static bool rwsem_optimistic_spin(struct rw_semaphore
*sem
)
456 static inline bool rwsem_has_spinner(struct rw_semaphore
*sem
)
463 * Wait until we successfully acquire the write lock
465 static inline struct rw_semaphore
*
466 __rwsem_down_write_failed_common(struct rw_semaphore
*sem
, int state
)
469 bool waiting
= true; /* any queued threads before us */
470 struct rwsem_waiter waiter
;
471 struct rw_semaphore
*ret
= sem
;
472 DEFINE_WAKE_Q(wake_q
);
474 /* undo write bias from down_write operation, stop active locking */
475 count
= atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS
, &sem
->count
);
477 /* do optimistic spinning and steal lock if possible */
478 if (rwsem_optimistic_spin(sem
))
482 * Optimistic spinning failed, proceed to the slowpath
483 * and block until we can acquire the sem.
485 waiter
.task
= current
;
486 waiter
.type
= RWSEM_WAITING_FOR_WRITE
;
488 raw_spin_lock_irq(&sem
->wait_lock
);
490 /* account for this before adding a new element to the list */
491 if (list_empty(&sem
->wait_list
))
494 list_add_tail(&waiter
.list
, &sem
->wait_list
);
496 /* we're now waiting on the lock, but no longer actively locking */
498 count
= atomic_long_read(&sem
->count
);
501 * If there were already threads queued before us and there are
502 * no active writers, the lock must be read owned; so we try to
503 * wake any read locks that were queued ahead of us.
505 if (count
> RWSEM_WAITING_BIAS
) {
506 __rwsem_mark_wake(sem
, RWSEM_WAKE_READERS
, &wake_q
);
508 * The wakeup is normally called _after_ the wait_lock
509 * is released, but given that we are proactively waking
510 * readers we can deal with the wake_q overhead as it is
511 * similar to releasing and taking the wait_lock again
512 * for attempting rwsem_try_write_lock().
517 * Reinitialize wake_q after use.
519 wake_q_init(&wake_q
);
523 count
= atomic_long_add_return(RWSEM_WAITING_BIAS
, &sem
->count
);
525 /* wait until we successfully acquire the lock */
526 set_current_state(state
);
528 if (rwsem_try_write_lock(count
, sem
))
530 raw_spin_unlock_irq(&sem
->wait_lock
);
532 /* Block until there are no active lockers. */
534 if (signal_pending_state(state
, current
))
538 set_current_state(state
);
539 } while ((count
= atomic_long_read(&sem
->count
)) & RWSEM_ACTIVE_MASK
);
541 raw_spin_lock_irq(&sem
->wait_lock
);
543 __set_current_state(TASK_RUNNING
);
544 list_del(&waiter
.list
);
545 raw_spin_unlock_irq(&sem
->wait_lock
);
550 __set_current_state(TASK_RUNNING
);
551 raw_spin_lock_irq(&sem
->wait_lock
);
552 list_del(&waiter
.list
);
553 if (list_empty(&sem
->wait_list
))
554 atomic_long_add(-RWSEM_WAITING_BIAS
, &sem
->count
);
556 __rwsem_mark_wake(sem
, RWSEM_WAKE_ANY
, &wake_q
);
557 raw_spin_unlock_irq(&sem
->wait_lock
);
560 return ERR_PTR(-EINTR
);
563 __visible
struct rw_semaphore
* __sched
564 rwsem_down_write_failed(struct rw_semaphore
*sem
)
566 return __rwsem_down_write_failed_common(sem
, TASK_UNINTERRUPTIBLE
);
568 EXPORT_SYMBOL(rwsem_down_write_failed
);
570 __visible
struct rw_semaphore
* __sched
571 rwsem_down_write_failed_killable(struct rw_semaphore
*sem
)
573 return __rwsem_down_write_failed_common(sem
, TASK_KILLABLE
);
575 EXPORT_SYMBOL(rwsem_down_write_failed_killable
);
578 * handle waking up a waiter on the semaphore
579 * - up_read/up_write has decremented the active part of count if we come here
582 struct rw_semaphore
*rwsem_wake(struct rw_semaphore
*sem
)
585 DEFINE_WAKE_Q(wake_q
);
588 * If a spinner is present, it is not necessary to do the wakeup.
589 * Try to do wakeup only if the trylock succeeds to minimize
590 * spinlock contention which may introduce too much delay in the
593 * spinning writer up_write/up_read caller
594 * --------------- -----------------------
595 * [S] osq_unlock() [L] osq
597 * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
599 * Here, it is important to make sure that there won't be a missed
600 * wakeup while the rwsem is free and the only spinning writer goes
601 * to sleep without taking the rwsem. Even when the spinning writer
602 * is just going to break out of the waiting loop, it will still do
603 * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
604 * rwsem_has_spinner() is true, it will guarantee at least one
605 * trylock attempt on the rwsem later on.
607 if (rwsem_has_spinner(sem
)) {
609 * The smp_rmb() here is to make sure that the spinner
610 * state is consulted before reading the wait_lock.
613 if (!raw_spin_trylock_irqsave(&sem
->wait_lock
, flags
))
617 raw_spin_lock_irqsave(&sem
->wait_lock
, flags
);
620 if (!list_empty(&sem
->wait_list
))
621 __rwsem_mark_wake(sem
, RWSEM_WAKE_ANY
, &wake_q
);
623 raw_spin_unlock_irqrestore(&sem
->wait_lock
, flags
);
628 EXPORT_SYMBOL(rwsem_wake
);
631 * downgrade a write lock into a read lock
632 * - caller incremented waiting part of count and discovered it still negative
633 * - just wake up any readers at the front of the queue
636 struct rw_semaphore
*rwsem_downgrade_wake(struct rw_semaphore
*sem
)
639 DEFINE_WAKE_Q(wake_q
);
641 raw_spin_lock_irqsave(&sem
->wait_lock
, flags
);
643 if (!list_empty(&sem
->wait_list
))
644 __rwsem_mark_wake(sem
, RWSEM_WAKE_READ_OWNED
, &wake_q
);
646 raw_spin_unlock_irqrestore(&sem
->wait_lock
, flags
);
651 EXPORT_SYMBOL(rwsem_downgrade_wake
);