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 X-Spam-Level: X-Spam-Status: No, score=-11.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 131EFCA9ECB for ; Wed, 30 Oct 2019 22:50:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D1B7A21924 for ; Wed, 30 Oct 2019 22:50:32 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="U0sS/BZZ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727912AbfJ3Wub (ORCPT ); Wed, 30 Oct 2019 18:50:31 -0400 Received: from mail-wm1-f66.google.com ([209.85.128.66]:36999 "EHLO mail-wm1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727905AbfJ3Wu3 (ORCPT ); Wed, 30 Oct 2019 18:50:29 -0400 Received: by mail-wm1-f66.google.com with SMTP id q130so3945688wme.2 for ; Wed, 30 Oct 2019 15:50:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=f5pv0TrDckCPKq/kpy/66V+Rj+yO2dx6a+ZGMx39OTk=; b=U0sS/BZZNsP6TKkYYN6B10usHzz5S2232rBMjOo8KfKq6KyHiKYTpvAbgVE7D8EeSY SVa8hTEtcpbOMAjqXM5wXgiB5Lv7QuXvcV3y5xfYctwDBS5/DzmXhMoKj8CAg7MVyQTl +m4RAoaytlo9rAE58wWfIrpQxfG6b5v61eZttegGq7EZzh1PHz0YP6ZkwhDloBI/T42T sVsxSqrrlMb4QWs4AmNK6xyPMILT90LUvJcBc/msOQF2+f8Sl1zHFhb56EE6U0lHCdiV yrObMW+p7g7GGVfyQOuyYkj/9z8NZ0B0907/7i5EyR0RbbirpmZ4mLqTVHR9a9K1MGEK uTqw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=f5pv0TrDckCPKq/kpy/66V+Rj+yO2dx6a+ZGMx39OTk=; b=cfxMBJw830J0xDafmJ6Sdg75nWHTNikeVbTnWnSFtRXrWW+XU0UpbfqkSfs1TEfOEa /h1VD10MKTZ9xh9hrhPUXNIZP56J5N8mWxEmykzpOvlGvWmuBACwh879COxLbyC3SLq9 bmAHy2nEdLlSADs8xdQx8ckgkuZMj9G65KN47MFeN6ZO4RInQgK+IpdXmZXKE1/XDTwg zP7s0LEClLWQ2ZEIsQ4i+2ckiNIHGrlBFgTluShKJ7klb21IKEAlfyxJ1DG6IRRLTOEb M7zyBoUhsJQlpoyxBepcJS47o8HhoAU0XLcluf/szDtxxavRgButCyaHXImkyJIgaFpa 8GKw== X-Gm-Message-State: APjAAAWYKP+FrP6ao9P5auJnTqTGch6HuR30NE2ZOT5QtZ6XAm18BqAF rJetp+jHGogd/IqpqECn+brSpf7AM/6GZ47ZSy1udw== X-Google-Smtp-Source: APXvYqwIuR/7aU9AfLmzsUpOEiGm8Sfd5QT+QzevSUMt1500kwG/j/o5S8PC06ZvL4JIe6bzRkQ/0BgFHrTFeeuGW8I= X-Received: by 2002:a1c:f60d:: with SMTP id w13mr1885289wmc.150.1572475826967; Wed, 30 Oct 2019 15:50:26 -0700 (PDT) MIME-Version: 1.0 References: <20191028174603.GA246917@google.com> <20191029113411.GP4643@worktop.programming.kicks-ass.net> <20191029115000.GA11194@google.com> In-Reply-To: <20191029115000.GA11194@google.com> From: Ram Muthiah Date: Wed, 30 Oct 2019 15:50:00 -0700 Message-ID: Subject: Re: NULL pointer dereference in pick_next_task_fair To: Quentin Perret Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, aaron.lwe@gmail.com, valentin.schneider@arm.com, mingo@kernel.org, pauld@redhat.com, jdesfossez@digitalocean.com, naravamudan@digitalocean.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, juri.lelli@redhat.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, kernel-team@android.com, john.stultz@linaro.org Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 29, 2019 at 4:50 AM 'Quentin Perret' via kernel-team wrote: > > On Tuesday 29 Oct 2019 at 12:34:11 (+0100), Peter Zijlstra wrote: > > On Mon, Oct 28, 2019 at 05:46:03PM +0000, Quentin Perret wrote: > > > The issue is very transient and relatively hard to reproduce. > > > > > > After digging a bit, the offending commit seems to be: > > > > > > 67692435c411 ("sched: Rework pick_next_task() slow-path") > > > > > > By 'offending' I mean that reverting it makes the issue go away. The > > > issue comes from the fact that pick_next_entity() returns a NULL se in > > > the 'simple' path of pick_next_task_fair(), which causes obvious > > > problems in the subsequent call to set_next_entity(). > > > > > > I'll dig more, but if anybody understands the issue in the meatime feel > > > free to send me a patch to try out :) > > > > Can you please see if this makes any difference? > > > > --- > > kernel/sched/core.c | 6 ++++-- > > kernel/sched/fair.c | 2 +- > > kernel/sched/idle.c | 3 +-- > > 3 files changed, 6 insertions(+), 5 deletions(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 7880f4f64d0e..abd2d4f80381 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -3922,8 +3922,10 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > > goto restart; > > > > /* Assumes fair_sched_class->next == idle_sched_class */ > > - if (unlikely(!p)) > > - p = idle_sched_class.pick_next_task(rq, prev, rf); > > + if (unlikely(!p)) { > > + prev->sched_class->put_prev_task(rq, prev, rf); > > + p = idle_sched_class.pick_next_task(rq, NULL, NULL); > > + } > > > > return p; > > } > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 83ab35e2374f..2aad94bb7165 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6820,7 +6820,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf > > simple: > > #endif > > if (prev) > > - put_prev_task(rq, prev); > > + prev->sched_class->put_prev_task(rq, prev, rf); > > > > do { > > se = pick_next_entity(cfs_rq, NULL); > > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > > index 8dad5aa600ea..e8dfc84f375a 100644 > > --- a/kernel/sched/idle.c > > +++ b/kernel/sched/idle.c > > @@ -390,8 +390,7 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf > > { > > struct task_struct *next = rq->idle; > > > > - if (prev) > > - put_prev_task(rq, prev); > > + WARN_ON_ONCE(prev || rf); > > > > set_next_task_idle(rq, next); > > > > > > Unfortunately the issue went into hiding this morning and I struggle to > reproduce it (this is turning bordeline nightmare now TBH). > > I'll try the patch once my reproducer is fixed :/ > > Thank you very much for the help, > Quentin > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. > Quentin and I were able to create a setup which reproduces the issue. Given this, I tried Peter's proposed fix and was still able to reproduce the issue unfortunately. Current patch is located here - https://android-review.googlesource.com/c/kernel/common/+/1153487 Our mitigation for this issue on the android-mainline branch has been to revert 67692435c411 ("sched: Rework pick_next_task() slow-path"). https://android-review.googlesource.com/c/kernel/common/+/1152564 I'll spend some time detailing repro steps next. I should be able to provide an update on those details early next week. We appreciate the help so far. Thanks, Ram