From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1E9DBC433EF for ; Fri, 1 Jul 2022 11:48:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235458AbiGALs4 (ORCPT ); Fri, 1 Jul 2022 07:48:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49376 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233057AbiGALsx (ORCPT ); Fri, 1 Jul 2022 07:48:53 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id EEB5479719 for ; Fri, 1 Jul 2022 04:48:51 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 083A0113E; Fri, 1 Jul 2022 04:48:52 -0700 (PDT) Received: from wubuntu (unknown [10.57.84.222]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 54BA13F66F; Fri, 1 Jul 2022 04:48:49 -0700 (PDT) Date: Fri, 1 Jul 2022 12:48:46 +0100 From: Qais Yousef To: Peter Zijlstra Cc: Satya Durga Srinivasu Prabhala , mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: fix rq lock recursion issue Message-ID: <20220701114846.42o2tkm5fqt325df@wubuntu> References: <20220624074240.13108-1-quic_satyap@quicinc.com> <20220630215310.wb3kab72tlh5pq2g@airbuntu> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/01/22 10:33, Peter Zijlstra wrote: > On Thu, Jun 30, 2022 at 10:53:10PM +0100, Qais Yousef wrote: > > Hi Satya > > > > On 06/24/22 00:42, Satya Durga Srinivasu Prabhala wrote: > > > Below recursion is observed in a rare scenario where __schedule() > > > takes rq lock, at around same time task's affinity is being changed, > > > bpf function for tracing sched_switch calls migrate_enabled(), > > > checks for affinity change (cpus_ptr != cpus_mask) lands into > > > __set_cpus_allowed_ptr which tries acquire rq lock and causing the > > > recursion bug. > > > > > > Fix the issue by switching to preempt_enable/disable() for non-RT > > > Kernels. > > > > Interesting bug. Thanks for the report. Unfortunately I can't see this being > > a fix as it just limits the bug visibility to PREEMPT_RT kernels, but won't fix > > anything, no? ie: Kernels compiled with PREEMPT_RT will still hit this failure. > > Worse, there's !RT stuff that grew to rely on the preemptible migrate > disable stuff, so this actively breaks things. > > > I'm curious how the race with set affinity is happening. I would have thought > > user space would get blocked as __schedule() will hold the rq lock. > > > > Do you have more details on that? > > Yeah, I'm not seeing how this works either, in order for > migrate_enable() to actually call __set_cpus_allowed_ptr(), it needs to > have done migrate_disable() *before* schedule, schedule() will then have > to call migrate_disable_swich(), and *then* migrate_enable() does this. > > However, if things are nicely balanced (as they should be), then > trace_call_bpf() using migrate_disable()/migrate_enable() should never > hit this path. > > If, OTOH, migrate_disable() was called prior to schedule() and we did do > migrate_disable_switch(), then it should be impossible for the > tracepoint/bpf stuff to reach p->migration_disabled == 0. I think it's worth to confirm which kernel Satya is on too. If it's GKI, then worth checking first this is actually reproducible on/applicable to mainline. Cheers -- Qais Yousef