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From: David Xu <davidxu@FreeBSD.org>
Date: Mon, 16 Jan 2012 04:39:10 +0000 (UTC)
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Subject: svn commit: r230194 - head/sys/kern
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Author: davidxu
Date: Mon Jan 16 04:39:10 2012
New Revision: 230194
URL: http://svn.freebsd.org/changeset/base/230194

Log:
  Eliminate branch and insert an explicit reader memory barrier to ensure
  that waiter bit is set before reading semaphore count.

Modified:
  head/sys/kern/kern_umtx.c

Modified: head/sys/kern/kern_umtx.c
==============================================================================
--- head/sys/kern/kern_umtx.c	Mon Jan 16 04:09:45 2012	(r230193)
+++ head/sys/kern/kern_umtx.c	Mon Jan 16 04:39:10 2012	(r230194)
@@ -2804,9 +2804,8 @@ do_sem_wait(struct thread *td, struct _u
 	umtxq_insert(uq);
 	umtxq_unlock(&uq->uq_key);
 
-	if (fuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters)) == 0)
-		casuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters), 0, 1);
-
+	casuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters), 0, 1);
+	rmb();
 	count = fuword32(__DEVOLATILE(uint32_t *, &sem->_count));
 	if (count != 0) {
 		umtxq_lock(&uq->uq_key);