Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758454Ab1COT17 (ORCPT ); Tue, 15 Mar 2011 15:27:59 -0400 Received: from mail4.comsite.net ([205.238.176.238]:28374 "EHLO mail4.comsite.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757762Ab1COT1R (ORCPT ); Tue, 15 Mar 2011 15:27:17 -0400 X-Default-Received-SPF: pass (skip=forwardok (res=PASS)) x-ip-name=71.22.127.106; Subject: [PATCH 3/4 v3] smp_call_function_many: handle concurrent clearing of mask To: akpm@linux-foundation.org, Mike Galbraith , Dimitri Sivanich , Tony Luck , Jan Beulich From: Milton Miller Cc: Peter Zijlstra , Anton Blanchard , xiaoguangrong@cn.fujitsu.com, mingo@elte.hu, jaxboe@fusionio.com, npiggin@gmail.com, rusty@rustcorp.com.au, torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com, benh@kernel.crashing.org, linux-kernel@vger.kernel.org Message-ID: In-Reply-To: References: <20110112150740.77dde58c@kryten> <1295288253.30950.280.camel@laptop> <1296145360.15234.234.camel@laptop> <1296458482.7889.175.camel@marge.simson.net> Date: Tue, 15 Mar 2011 13:27:17 -0600 X-Originating-IP: 71.22.127.106 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3289 Lines: 81 Mike Galbraith reported finding a lockup ("perma-spin bug") where the cpumask passed to smp_call_function_many was cleared by other cpu(s) while a cpu was preparing its call_data block, resulting in no cpu to clear the last ref and unlock the block. Having cpus clear their bit asynchronously could be useful on a mask of cpus that might have a translation context, or cpus that need a push to complete an rcu window. Instead of adding a BUG_ON and requiring yet another cpumask copy, just detect the race and handle it. Note: arch_send_call_function_ipi_mask must still handle an empty cpumask because the data block is globally visible before the that arch callback is made. And (obviously) there are no guarantees to which cpus are notified if the mask is changed during the call; only cpus that were online and had their mask bit set during the whole call are guaranteed to be called. Reported-by: Mike Galbraith Reported-by: Jan Beulich Signed-off-by: Milton Miller --- v3: try to clarify which mask in comment v2: rediff for v2 of call_function_many: fix list delete vs add race The arch code not expecting the race to empty the mask is the cause of https://bugzilla.kernel.org/show_bug.cgi?id=23042 that Andrew pointed out. Index: common/kernel/smp.c =================================================================== --- common.orig/kernel/smp.c 2011-03-15 05:22:26.000000000 -0500 +++ common/kernel/smp.c 2011-03-15 06:22:26.000000000 -0500 @@ -450,7 +450,7 @@ void smp_call_function_many(const struct { struct call_function_data *data; unsigned long flags; - int cpu, next_cpu, this_cpu = smp_processor_id(); + int refs, cpu, next_cpu, this_cpu = smp_processor_id(); /* * Can deadlock when called with interrupts disabled. @@ -461,7 +461,7 @@ void smp_call_function_many(const struct WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled() && !oops_in_progress && !early_boot_irqs_disabled); - /* So, what's a CPU they want? Ignoring this one. */ + /* Try to fastpath. So, what's a CPU they want? Ignoring this one. */ cpu = cpumask_first_and(mask, cpu_online_mask); if (cpu == this_cpu) cpu = cpumask_next_and(cpu, mask, cpu_online_mask); @@ -519,6 +519,13 @@ void smp_call_function_many(const struct /* We rely on the "and" being processed before the store */ cpumask_and(data->cpumask, mask, cpu_online_mask); cpumask_clear_cpu(this_cpu, data->cpumask); + refs = cpumask_weight(data->cpumask); + + /* Some callers race with other cpus changing the passed mask */ + if (unlikely(!refs)) { + csd_unlock(&data->csd); + return; + } raw_spin_lock_irqsave(&call_function.lock, flags); /* @@ -532,7 +539,7 @@ void smp_call_function_many(const struct * to the cpumask before this write to refs, which indicates * data is on the list and is ready to be processed. */ - atomic_set(&data->refs, cpumask_weight(data->cpumask)); + atomic_set(&data->refs, refs); raw_spin_unlock_irqrestore(&call_function.lock, flags); /* -- 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/