Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756284Ab2JDGQ4 (ORCPT ); Thu, 4 Oct 2012 02:16:56 -0400 Received: from e28smtp05.in.ibm.com ([122.248.162.5]:52231 "EHLO e28smtp05.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755183Ab2JDGQy (ORCPT ); Thu, 4 Oct 2012 02:16:54 -0400 Message-ID: <506D29A7.1000805@linux.vnet.ibm.com> Date: Thu, 04 Oct 2012 11:46:07 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Andrew Morton CC: Jiri Kosina , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , "Paul E. McKenney" , Christoph Lameter , Pekka Enberg , "Paul E. McKenney" , Josh Triplett , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] CPU hotplug, debug: Detect imbalance between get_online_cpus() and put_online_cpus() References: <20121002170149.GC2465@linux.vnet.ibm.com> <20121002233138.GD2465@linux.vnet.ibm.com> <20121003001530.GF2465@linux.vnet.ibm.com> <506C2E02.9080804@linux.vnet.ibm.com> <506C3535.3070401@linux.vnet.ibm.com> <20121003141311.09fb3ffc.akpm@linux-foundation.org> In-Reply-To: <20121003141311.09fb3ffc.akpm@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit x-cbid: 12100406-8256-0000-0000-0000046ED0EF Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4562 Lines: 119 On 10/04/2012 02:43 AM, Andrew Morton wrote: > On Wed, 03 Oct 2012 18:23:09 +0530 > "Srivatsa S. Bhat" wrote: > >> The synchronization between CPU hotplug readers and writers is achieved by >> means of refcounting, safe-guarded by the cpu_hotplug.lock. >> >> get_online_cpus() increments the refcount, whereas put_online_cpus() decrements >> it. If we ever hit an imbalance between the two, we end up compromising the >> guarantees of the hotplug synchronization i.e, for example, an extra call to >> put_online_cpus() can end up allowing a hotplug reader to execute concurrently with >> a hotplug writer. So, add a BUG_ON() in put_online_cpus() to detect such cases >> where the refcount can go negative. >> >> Signed-off-by: Srivatsa S. Bhat >> --- >> >> kernel/cpu.c | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/kernel/cpu.c b/kernel/cpu.c >> index f560598..00d29bc 100644 >> --- a/kernel/cpu.c >> +++ b/kernel/cpu.c >> @@ -80,6 +80,7 @@ void put_online_cpus(void) >> if (cpu_hotplug.active_writer == current) >> return; >> mutex_lock(&cpu_hotplug.lock); >> + BUG_ON(cpu_hotplug.refcount == 0); >> if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) >> wake_up_process(cpu_hotplug.active_writer); >> mutex_unlock(&cpu_hotplug.lock); > > I think calling BUG() here is a bit harsh. We should only do that if > there's a risk to proceeding: a risk of data loss, a reduced ability to > analyse the underlying bug, etc. > > But a cpu-hotplug locking imbalance is a really really really minor > problem! So how about we emit a warning then try to fix things up? That would be better indeed, thanks! > This should increase the chance that the machine will keep running and > so will increase the chance that a user will be able to report the bug > to us. > Yep, sounds good. > > --- a/kernel/cpu.c~cpu-hotplug-debug-detect-imbalance-between-get_online_cpus-and-put_online_cpus-fix > +++ a/kernel/cpu.c > @@ -80,9 +80,12 @@ void put_online_cpus(void) > if (cpu_hotplug.active_writer == current) > return; > mutex_lock(&cpu_hotplug.lock); > - BUG_ON(cpu_hotplug.refcount == 0); > - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) > - wake_up_process(cpu_hotplug.active_writer); > + if (!--cpu_hotplug.refcount) { This won't catch it. We'll enter this 'if' condition only when cpu_hotplug.refcount was decremented to zero. We'll miss out the case when it went negative (which we intended to detect). > + if (WARN_ON(cpu_hotplug.refcount == -1)) > + cpu_hotplug.refcount++; /* try to fix things up */ > + if (unlikely(cpu_hotplug.active_writer)) > + wake_up_process(cpu_hotplug.active_writer); > + } > mutex_unlock(&cpu_hotplug.lock); > > } So how about something like below: ------------------------------------------------------> From: Srivatsa S. Bhat Subject: [PATCH] CPU hotplug, debug: Detect imbalance between get_online_cpus() and put_online_cpus() The synchronization between CPU hotplug readers and writers is achieved by means of refcounting, safe-guarded by the cpu_hotplug.lock. get_online_cpus() increments the refcount, whereas put_online_cpus() decrements it. If we ever hit an imbalance between the two, we end up compromising the guarantees of the hotplug synchronization i.e, for example, an extra call to put_online_cpus() can end up allowing a hotplug reader to execute concurrently with a hotplug writer. So, add a WARN_ON() in put_online_cpus() to detect such cases where the refcount can go negative, and also attempt to fix it up, so that we can continue to run. Signed-off-by: Srivatsa S. Bhat --- kernel/cpu.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/cpu.c b/kernel/cpu.c index f560598..42bd331 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -80,6 +80,10 @@ void put_online_cpus(void) if (cpu_hotplug.active_writer == current) return; mutex_lock(&cpu_hotplug.lock); + + if (WARN_ON(!cpu_hotplug.refcount)) + cpu_hotplug.refcount++; /* try to fix things up */ + if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) wake_up_process(cpu_hotplug.active_writer); mutex_unlock(&cpu_hotplug.lock); -- 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/