Date: Tue, 9 Feb 2010 01:19:10 +0000 (UTC) From: David Xu <davidxu@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-7@freebsd.org Subject: svn commit: r203694 - stable/7/sys/kern Message-ID: <201002090119.o191JAj4013460@svn.freebsd.org>
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Author: davidxu Date: Tue Feb 9 01:19:10 2010 New Revision: 203694 URL: http://svn.freebsd.org/changeset/base/203694 Log: MFC r203414: 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. Approved by: re (kib) Modified: stable/7/sys/kern/kern_umtx.c Directory Properties: stable/7/sys/ (props changed) stable/7/sys/cddl/contrib/opensolaris/ (props changed) stable/7/sys/contrib/dev/acpica/ (props changed) stable/7/sys/contrib/pf/ (props changed) Modified: stable/7/sys/kern/kern_umtx.c ============================================================================== --- stable/7/sys/kern/kern_umtx.c Tue Feb 9 00:38:40 2010 (r203693) +++ stable/7/sys/kern/kern_umtx.c Tue Feb 9 01:19:10 2010 (r203694) @@ -2463,6 +2463,12 @@ 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); @@ -2595,6 +2601,12 @@ 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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