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Date: Wed, 30 Jun 2010 12:51:21 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Manfred Spraul <manfred@colorfullife.com>
Cc: Christoph Lameter <cl@linux-foundation.org>,
       Pekka Enberg <penberg@cs.helsinki.fi>, linux-mm@kvack.org,
       Nick Piggin <npiggin@suse.de>, Matt Mackall <mpm@selenic.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [S+Q 01/16] [PATCH] ipc/sem.c: Bugfix for semop() not reporting
 successful operation
Message-Id: <20100630125121.6b076f1b.akpm@linux-foundation.org>
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On Wed, 30 Jun 2010 21:38:43 +0200
Manfred Spraul <manfred@colorfullife.com> wrote:

> Hi Andrew,
> 
> On 06/29/2010 09:08 PM, Andrew Morton wrote:
> > On Tue, 29 Jun 2010 10:42:42 -0500 (CDT)
> > Christoph Lameter<cl@linux-foundation.org>  wrote:
> >
> >    
> >> This is a patch from Manfred. Required to make 2.6.35-rc3 work.
> >>
> >>      
> > My current version of the patch is below.
> >
> > I believe that Luca has still seen problems with this patch applied so
> > its current status is "stuck, awaiting developments".
> >
> > Is that a correct determination?
> >    
> 
> I would propose that you forward a patch to Linus - either the one you 
> have in your tree or the v2 that I've just posted.

OK, I added the incremental change:

--- a/ipc/sem.c~ipc-semc-bugfix-for-semop-not-reporting-successful-operation-update
+++ a/ipc/sem.c
@@ -1440,7 +1440,14 @@ SYSCALL_DEFINE4(semtimedop, int, semid, 
 
 	if (error != -EINTR) {
 		/* fast path: update_queue already obtained all requested
-		 * resources */
+		 * resources.
+		 * Perform a smp_mb(): User space could assume that semop()
+		 * is a memory barrier: Without the mb(), the cpu could
+		 * speculatively read in user space stale data that was
+		 * overwritten by the previous owner of the semaphore.
+		 */
+		smp_mb();
+
 		goto out_free;
 	}
 
_

> With stock 2.6.35-rc3, my semtimedop() stress tests produces an oops or 
> an invalid return value (i.e.:semtimedop() returns with "1") within a 
> fraction of a second.
> 
> With either of the patches applied, my test apps show the expected behavior.

OK, I'll queue it up.