Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758621AbZDKRtS (ORCPT ); Sat, 11 Apr 2009 13:49:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758358AbZDKRtI (ORCPT ); Sat, 11 Apr 2009 13:49:08 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:35733 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757922AbZDKRtF (ORCPT ); Sat, 11 Apr 2009 13:49:05 -0400 Date: Sat, 11 Apr 2009 10:49:05 -0700 From: "Paul E. McKenney" To: Jan Blunck Cc: Nick Piggin , Linux-Kernel Mailinglist Subject: Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well Message-ID: <20090411174905.GH6822@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090411141754.45F7B16080@e179.suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090411141754.45F7B16080@e179.suse.de> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1447 Lines: 43 On Fri, Apr 10, 2009 at 06:13:57PM +0200, Jan Blunck wrote: > I think it is wrong to unconditionally take the lock before calling > atomic_dec_and_test() in _atomic_dec_and_lock(). This will deadlock in > situation where it is known that the counter will not reach zero (e.g. holding > another reference to the same object) but the lock is already taken. The thought of calling _atomic_dec_and_lock() when you already hold the lock really really scares me. Could you please give an example where you need to do this? Thanx, Paul > Signed-off-by: Jan Blunck > --- > lib/dec_and_lock.c | 3 +-- > 1 files changed, 1 insertions(+), 2 deletions(-) > > diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c > index a65c314..e73822a 100644 > --- a/lib/dec_and_lock.c > +++ b/lib/dec_and_lock.c > @@ -19,11 +19,10 @@ > */ > int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock) > { > -#ifdef CONFIG_SMP > /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */ > if (atomic_add_unless(atomic, -1, 1)) > return 0; > -#endif > + > /* Otherwise do it the slow way */ > spin_lock(lock); > if (atomic_dec_and_test(atomic)) > -- > 1.6.0.2 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/