Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755211Ab1FONvF (ORCPT ); Wed, 15 Jun 2011 09:51:05 -0400 Received: from relay2.sgi.com ([192.48.179.30]:44401 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753925Ab1FONvD (ORCPT ); Wed, 15 Jun 2011 09:51:03 -0400 Date: Wed, 15 Jun 2011 08:52:13 -0500 From: Cliff Wickman To: Rakib Mullick Cc: linux-kernel@vger.kernel.org, mingo@elte.hu Subject: Re: [PATCH 1 of 6] x86, UV: smp_processor_id in a preemptable region Message-ID: <20110615135213.GA29493@sgi.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2696 Lines: 68 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. It is possible that the found per-cpu structure could differ from cpu to cpu. But if this tunable is thus caused to be set too high, a 'throttling' upper bound will not be enforced. No other harm is done. But yes, I should have noted that an ugly DEBUG_PREEMPT warning will be the worst effect. -Cliff > > > ? ? ? ?bcp = &per_cpu(bau_control, smp_processor_id()); > > + ? ? ? preempt_enable_no_resched(); > > > > ? ? ? ?ret = parse_tunables_write(bcp, instr, count); > > ? ? ? ?if (ret) > > -- > > 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/ > > > -- > 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/ -- Cliff Wickman SGI cpw@sgi.com (651) 683-3824 -- 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/