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After busied the lock, re-read state word before checking waiters flag,

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otherwise, the waiters bit may not be set and a wakeup is lost.

Submitted by:	justin.teller at gmail dot com
MFC after:	3 days
This commit is contained in:
David Xu 2010-02-03 03:56:32 +00:00
parent e70f20b0e8
commit 676e6574a1
1 changed files with 12 additions and 0 deletions
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12
sys/kern/kern_umtx.c
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@ -2526,6 +2526,12 @@ do_rw_rdlock(struct thread *td, struct urwlock *rwlock, long fflag, int timo)
umtxq_busy(&uq->uq_key);
umtxq_unlock(&uq->uq_key);
/*
* re-read the state, in case it changed between the try-lock above
* and the check below
*/
state = fuword32(__DEVOLATILE(int32_t *, &rwlock->rw_state));
/* set read contention bit */
while ((state & wrflags) && !(state & URWLOCK_READ_WAITERS)) {
oldstate = casuword32(&rwlock->rw_state, state, state | URWLOCK_READ_WAITERS);
@ -2658,6 +2664,12 @@ do_rw_wrlock(struct thread *td, struct urwlock *rwlock, int timo)
umtxq_busy(&uq->uq_key);
umtxq_unlock(&uq->uq_key);
/*
* re-read the state, in case it changed between the try-lock above
* and the check below
*/
state = fuword32(__DEVOLATILE(int32_t *, &rwlock->rw_state));
while (((state & URWLOCK_WRITE_OWNER) || URWLOCK_READER_COUNT(state) != 0) &&
(state & URWLOCK_WRITE_WAITERS) == 0) {
oldstate = casuword32(&rwlock->rw_state, state, state | URWLOCK_WRITE_WAITERS);