From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751910AbdG1OFt (ORCPT ); Fri, 28 Jul 2017 10:05:49 -0400 Received: from mx2.suse.de ([195.135.220.15]:44620 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751628AbdG1OFr (ORCPT ); Fri, 28 Jul 2017 10:05:47 -0400 Subject: Re: [PATCH v2] cpuset: fix a deadlock due to incomplete patching of cpusets_enabled() To: Peter Zijlstra Cc: Dima Zavin , Christopher Lameter , Li Zefan , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Cliff Spradlin , Mel Gorman References: <20170727164608.12701-1-dmitriyz@waymo.com> <41954034-9de1-de8e-f915-51a4b0334f98@suse.cz> <20170728093047.ykgbufjj74xa5x3r@hirez.programming.kicks-ass.net> From: Vlastimil Babka Message-ID: <9e14ff85-1680-e76d-1b71-22301c16c286@suse.cz> Date: Fri, 28 Jul 2017 16:05:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170728093047.ykgbufjj74xa5x3r@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/28/2017 11:30 AM, Peter Zijlstra wrote: > On Fri, Jul 28, 2017 at 09:45:16AM +0200, Vlastimil Babka wrote: >> [+CC PeterZ] >> >> On 07/27/2017 06:46 PM, Dima Zavin wrote: >>> In codepaths that use the begin/retry interface for reading >>> mems_allowed_seq with irqs disabled, there exists a race condition that >>> stalls the patch process after only modifying a subset of the >>> static_branch call sites. >>> >>> This problem manifested itself as a dead lock in the slub >>> allocator, inside get_any_partial. The loop reads >>> mems_allowed_seq value (via read_mems_allowed_begin), >>> performs the defrag operation, and then verifies the consistency >>> of mem_allowed via the read_mems_allowed_retry and the cookie >>> returned by xxx_begin. The issue here is that both begin and retry >>> first check if cpusets are enabled via cpusets_enabled() static branch. >>> This branch can be rewritted dynamically (via cpuset_inc) if a new >>> cpuset is created. The x86 jump label code fully synchronizes across >>> all CPUs for every entry it rewrites. If it rewrites only one of the >>> callsites (specifically the one in read_mems_allowed_retry) and then >>> waits for the smp_call_function(do_sync_core) to complete while a CPU is >>> inside the begin/retry section with IRQs off and the mems_allowed value >>> is changed, we can hang. This is because begin() will always return 0 >>> (since it wasn't patched yet) while retry() will test the 0 against >>> the actual value of the seq counter. >> >> Hm I wonder if there are other static branch users potentially having >> similar problem. Then it would be best to fix this at static branch >> level. Any idea, Peter? An inelegant solution would be to have indicate >> static_branch_(un)likely() callsites ordering for the patching. I.e. >> here we would make sure that read_mems_allowed_begin() callsites are >> patched before read_mems_allowed_retry() when enabling the static key, >> and the opposite order when disabling the static key. > > I'm not aware of any other sure ordering requirements. But you can > manually create this order by using 2 static keys. Then flip them in the > desired order. Right, thanks for the suggestion. I think that would be preferable to complicating the cookie handling. Add a new key next to cpusets_enabled_key, let's say "cpusets_enabled_pre_key". Make read_mems_allowed_begin() check this key instead of cpusets_enabled(). Change cpuset_inc/dec to inc/dec also this new key in the right order and that should be it. Dima, can you try that or should I? Thanks, Vlastimil > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org >