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sx: change sunlock to wake waiters up if it locked sleepq

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sleepq is only locked if the curhtread is the last reader. By the time
the lock gets acquired new ones could have arrived. The previous code
would unlock and loop back. This results spurious relocking of sleepq.

This is a step towards xadd-based unlock routine.
This commit is contained in:
Mateusz Guzik 2017-11-25 20:13:50 +00:00
parent 93118b62f9
commit cec1747322
1 changed files with 21 additions and 20 deletions
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41
sys/kern/kern_sx.c
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@ -1168,45 +1168,46 @@ static void __noinline
_sx_sunlock_hard(struct sx *sx, uintptr_t x LOCK_FILE_LINE_ARG_DEF)
{
int wakeup_swapper;
uintptr_t setx;
if (SCHEDULER_STOPPED())
return;
if (_sx_sunlock_try(sx, &x))
goto out_lockstat;
/*
* At this point, there should just be one sharer with
* exclusive waiters.
*/
MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
sleepq_lock(&sx->lock_object);
x = SX_READ_VALUE(sx);
for (;;) {
if (_sx_sunlock_try(sx, &x))
break;
/*
* At this point, there should just be one sharer with
* exclusive waiters.
*/
MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
sleepq_lock(&sx->lock_object);
MPASS(x & SX_LOCK_EXCLUSIVE_WAITERS);
MPASS(!(x & SX_LOCK_SHARED_WAITERS));
/*
* Wake up semantic here is quite simple:
* Just wake up all the exclusive waiters.
* Note that the state of the lock could have changed,
* so if it fails loop back and retry.
*/
if (!atomic_cmpset_rel_ptr(&sx->sx_lock,
SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
SX_LOCK_UNLOCKED)) {
sleepq_release(&sx->lock_object);
x = SX_READ_VALUE(sx);
setx = x - SX_ONE_SHARER;
setx &= ~SX_LOCK_EXCLUSIVE_WAITERS;
if (!atomic_fcmpset_rel_ptr(&sx->sx_lock, &x, setx))
continue;
}
if (LOCK_LOG_TEST(&sx->lock_object, 0))
CTR2(KTR_LOCK, "%s: %p waking up all thread on"
"exclusive queue", __func__, sx);
wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
0, SQ_EXCLUSIVE_QUEUE);
sleepq_release(&sx->lock_object);
if (wakeup_swapper)
kick_proc0();
break;
}
sleepq_release(&sx->lock_object);
if (wakeup_swapper)
kick_proc0();
out_lockstat:
LOCKSTAT_PROFILE_RELEASE_RWLOCK(sx__release, sx, LOCKSTAT_READER);
}