Date: Wed, 3 Feb 2010 03:56:33 +0000 (UTC) From: David Xu <davidxu@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r203414 - head/sys/kern Message-ID: <201002030356.o133uXks069344@svn.freebsd.org>
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Author: davidxu Date: Wed Feb 3 03:56:32 2010 New Revision: 203414 URL: http://svn.freebsd.org/changeset/base/203414 Log: After busied the lock, re-read state word before checking waiters flag, 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 Modified: head/sys/kern/kern_umtx.c Modified: head/sys/kern/kern_umtx.c ============================================================================== --- head/sys/kern/kern_umtx.c Wed Feb 3 03:26:00 2010 (r203413) +++ head/sys/kern/kern_umtx.c Wed Feb 3 03:56:32 2010 (r203414) @@ -2526,6 +2526,12 @@ do_rw_rdlock(struct thread *td, struct u 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 u 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);
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