Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751861Ab1FOQjS (ORCPT ); Wed, 15 Jun 2011 12:39:18 -0400 Received: from relay3.sgi.com ([192.48.152.1]:38521 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751407Ab1FOQjR (ORCPT ); Wed, 15 Jun 2011 12:39:17 -0400 Date: Wed, 15 Jun 2011 11:40:27 -0500 From: Cliff Wickman To: Ingo Molnar Cc: 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: <20110615164027.GA32641@sgi.com> References: <20110615135213.GA29493@sgi.com> <20110615155445.GC4096@elte.hu> <20110615160743.GA32008@sgi.com> <20110615161518.GA24948@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110615161518.GA24948@elte.hu> 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: 3028 Lines: 78 On Wed, Jun 15, 2011 at 06:15:18PM +0200, Ingo Molnar wrote: > > * Cliff Wickman wrote: > > > Hi Ingo, > > > > On Wed, Jun 15, 2011 at 05:54:45PM +0200, Ingo Molnar wrote: > > > > > > * 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. > > > > You're right of course. But we don't support CPU hotplug on the UV > > hardware. There are enhancements needed in both the BIOS and Linux > > (BAU and GRU among them). They are on our work queue. > > But here you put in yet another roadblock. So would you say I should really widen the scope of the non-preemptible region to include everything done with the results of that call to smp_processor_id()? Which in this case is the call to parse_tunables_write(). Like this: preempt_disable(); bcp = &per_cpu(bau_control, smp_processor_id()); ret = parse_tunables_write(bcp, instr, count); preempt_enable_no_resched(); -Cliff -- 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/