Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755035Ab2JCMx7 (ORCPT ); Wed, 3 Oct 2012 08:53:59 -0400 Received: from e23smtp06.au.ibm.com ([202.81.31.148]:54017 "EHLO e23smtp06.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754997Ab2JCMx5 (ORCPT ); Wed, 3 Oct 2012 08:53:57 -0400 Message-ID: <506C3535.3070401@linux.vnet.ibm.com> Date: Wed, 03 Oct 2012 18:23:09 +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: Jiri Kosina , Thomas Gleixner , "akpm@linux-foundation.org" , Ingo Molnar , Peter Zijlstra CC: "Paul E. McKenney" , Christoph Lameter , Pekka Enberg , "Paul E. McKenney" , Josh Triplett , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [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> In-Reply-To: <506C2E02.9080804@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit x-cbid: 12100312-7014-0000-0000-000001F9F3E8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2350 Lines: 60 On 10/03/2012 05:52 PM, Srivatsa S. Bhat wrote: > On 10/03/2012 03:16 PM, Jiri Kosina wrote: >> On Wed, 3 Oct 2012, Jiri Kosina wrote: >> >>> Good question. I believe it should be safe to drop slab_mutex earlier, as >>> cachep has already been unlinked. I am adding slab people and linux-mm to >>> CC (the whole thread on LKML can be found at >>> https://lkml.org/lkml/2012/10/2/296 for reference). >>> [...] > > But, I'm also quite surprised that the put_online_cpus() code as it stands today > doesn't have any checks for the refcount going negative. I believe that such a > check would be valuable to help catch cases where we might end up inadvertently > causing an imbalance between get_online_cpus() and put_online_cpus(). I'll post > that as a separate patch. > ----------------------------------- 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 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); -- 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/