From owner-svn-src-all@FreeBSD.ORG  Mon Nov 22 02:42:03 2010
Return-Path: <owner-svn-src-all@FreeBSD.ORG>
Delivered-To: svn-src-all@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 18829106564A;
	Mon, 22 Nov 2010 02:42:03 +0000 (UTC)
	(envelope-from davidxu@FreeBSD.org)
Received: from svn.freebsd.org (svn.freebsd.org [IPv6:2001:4f8:fff6::2c])
	by mx1.freebsd.org (Postfix) with ESMTP id 0759A8FC16;
	Mon, 22 Nov 2010 02:42:03 +0000 (UTC)
Received: from svn.freebsd.org (localhost [127.0.0.1])
	by svn.freebsd.org (8.14.3/8.14.3) with ESMTP id oAM2g25U075525;
	Mon, 22 Nov 2010 02:42:02 GMT (envelope-from davidxu@svn.freebsd.org)
Received: (from davidxu@localhost)
	by svn.freebsd.org (8.14.3/8.14.3/Submit) id oAM2g2mE075523;
	Mon, 22 Nov 2010 02:42:02 GMT (envelope-from davidxu@svn.freebsd.org)
Message-Id: <201011220242.oAM2g2mE075523@svn.freebsd.org>
From: David Xu <davidxu@FreeBSD.org>
Date: Mon, 22 Nov 2010 02:42:02 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
	svn-src-head@freebsd.org
X-SVN-Group: head
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Cc: 
Subject: svn commit: r215652 - head/sys/kern
X-BeenThere: svn-src-all@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: "SVN commit messages for the entire src tree \(except for &quot;
	user&quot; and &quot; projects&quot; \)" <svn-src-all.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/svn-src-all>,
	<mailto:svn-src-all-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/svn-src-all>
List-Post: <mailto:svn-src-all@freebsd.org>
List-Help: <mailto:svn-src-all-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/svn-src-all>,
	<mailto:svn-src-all-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 22 Nov 2010 02:42:03 -0000

Author: davidxu
Date: Mon Nov 22 02:42:02 2010
New Revision: 215652
URL: http://svn.freebsd.org/changeset/base/215652

Log:
  Use atomic instruction to set _has_writer, otherwise there is a race
  causes userland to not wake up a thread sleeping in kernel.
  
  MFC after: 3 days

Modified:
  head/sys/kern/kern_umtx.c

Modified: head/sys/kern/kern_umtx.c
==============================================================================
--- head/sys/kern/kern_umtx.c	Mon Nov 22 01:28:29 2010	(r215651)
+++ head/sys/kern/kern_umtx.c	Mon Nov 22 02:42:02 2010	(r215652)
@@ -2839,7 +2839,8 @@ do_sem_wait(struct thread *td, struct _u
 	umtxq_insert(uq);
 	umtxq_unlock(&uq->uq_key);
 
-	suword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters), 1);
+	if (fuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters)) == 0)
+		casuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters), 0, 1);
 
 	count = fuword32(__DEVOLATILE(uint32_t *, &sem->_count));
 	if (count != 0) {