Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751726AbdC0SDJ (ORCPT ); Mon, 27 Mar 2017 14:03:09 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:39118 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751353AbdC0SC7 (ORCPT ); Mon, 27 Mar 2017 14:02:59 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org B358E607E7 Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=jhugo@codeaurora.org Subject: Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline To: paulmck@linux.vnet.ibm.com References: <20170326232843.GA3637@linux.vnet.ibm.com> Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, pprakash@codeaurora.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Jens Axboe , Sebastian Andrzej Siewior , Thomas Gleixner , Richard Cochran , Boris Ostrovsky , Richard Weinberger From: Jeffrey Hugo Message-ID: Date: Mon, 27 Mar 2017 12:02:27 -0600 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170326232843.GA3637@linux.vnet.ibm.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2095 Lines: 53 Hi Paul. Thanks for the quick reply. On 3/26/2017 5:28 PM, Paul E. McKenney wrote: > On Sun, Mar 26, 2017 at 05:10:40PM -0600, Jeffrey Hugo wrote: >> It is a race between this work running, and the cpu offline processing. > > One quick way to test this assumption is to build a kernel with Kconfig > options CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y. This will > cause call_rcu_sched() to queue the work to a kthread, which can migrate > to some other CPU. If your analysis is correct, this should avoid > the deadlock. (Note that the deadlock should be fixed in any case, > just a diagnostic assumption-check procedure.) I enabled CONFIG_RCU_EXPERT=y, CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y in my build. I've only had time so far to do one test run however the issue reproduced, but it took a fair bit longer to do so. An initial look at the data indicates that the work is still not running. An odd observation, the two threads are no longer blocked on the same queue, but different ones. Let me look at this more and see what is going on now. >> What is the opinion of the domain experts? > > I do hope that we can come up with a better fix. No offense intended, > as coming up with -any- fix in the CPU-hotplug domain is not to be > denigrated, but this looks to be at vest quite fragile. > > Thanx, Paul > None taken. I'm not particularly attached to the current fix. I agree, it does appear to be quite fragile. I'm still not sure what a better solution would be though. Maybe the RCU framework flushes the work somehow during cpu offline? It would need to ensure further work is not queued after that point, which seems like it might be tricky to synchronize. I don't know enough about the working of RCU to even attempt to implement that. In any case, it seem like some more analysis is needed based on the latest data. -- Jeffrey Hugo Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.