Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755973Ab1FOPyu (ORCPT ); Wed, 15 Jun 2011 11:54:50 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:52905 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755191Ab1FOPyt (ORCPT ); Wed, 15 Jun 2011 11:54:49 -0400 Date: Wed, 15 Jun 2011 17:54:45 +0200 From: Ingo Molnar To: Cliff Wickman Cc: Rakib Mullick , linux-kernel@vger.kernel.org, Thomas Gleixner , "H. Peter Anvin" Subject: Re: [PATCH 1 of 6] x86, UV: smp_processor_id in a preemptable region Message-ID: <20110615155445.GC4096@elte.hu> References: <20110615135213.GA29493@sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110615135213.GA29493@sgi.com> User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.3.1 -2.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1893 Lines: 49 * Cliff Wickman wrote: > On Wed, Jun 15, 2011 at 12:05:17PM +0600, Rakib Mullick wrote: > > On Wed, Jun 15, 2011 at 5:06 AM, Cliff Wickman wrote: > > > From: Cliff Wickman > > > > > > Calling smp_processor_id() from within a preemptable region will issue > > > a warning if DEBUG_PREEMPT is set. > > > > > > Diffed against 3.0.0-rc3 > > > > > > Signed-off-by: Cliff Wickman > > > --- > > > ?arch/x86/platform/uv/tlb_uv.c | ? ?2 ++ > > > ?1 file changed, 2 insertions(+) > > > > > > Index: linux/arch/x86/platform/uv/tlb_uv.c > > > =================================================================== > > > --- linux.orig/arch/x86/platform/uv/tlb_uv.c > > > +++ linux/arch/x86/platform/uv/tlb_uv.c > > > @@ -1334,7 +1334,9 @@ static ssize_t tunables_write(struct fil > > > > > > ? ? ? ?instr[count] = '\0'; > > > > > > + ? ? ? preempt_disable(); /* avoid DEBUG_PREEMPT warning */ > > > > I think above code comment, "avoid DEBUG_PREEMPT warning" should be to > > something more meaningful. It's a BUG, if smp_processor_id() is called > > within preemptible context. So, we don't want to hit that BUG. > > I agree that calling smp_processor_id() within a preemptible context is > going to produce unpredictable results. In this particular case we just > need a valid cpu number so that we can find a per-cpu structure. > That structure contains a reasonable (sanity-checking) limit to the value > of the tunable that is being written. So what happens if the code gets preempted away and this CPU is hotplugged away? You'll reference a CPU ID that does not exist anymore. Thanks, Ingo -- 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/