Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 6 Sep 2022 22:50:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>,
        Jing-Ting Wu <jing-ting.wu@mediatek.com>,
        Mukesh Ojha <quic_mojha@quicinc.com>,
        Valentin Schneider <vschneid@redhat.com>,
        wsd_upstream@mediatek.com, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, Jonathan.JMChen@mediatek.com,
        "chris.redpath@arm.com" <chris.redpath@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Vincent Donnefort <vdonnefort@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Christian Brauner <brauner@kernel.org>,
        cgroups@vger.kernel.org, lixiong.liu@mediatek.com,
        wenju.xu@mediatek.com
Subject: Re: BUG: HANG_DETECT waiting for migration_cpu_stop() complete
Message-ID: <YxeylMvgc/wKcJqk@hirez.programming.kicks-ass.net>
References: <88b2910181bda955ac46011b695c53f7da39ac47.camel@mediatek.com>
 <b605c3ec-94ab-a55f-5825-9b370d77ecf3@quicinc.com>
 <203d4614c1b2a498a240ace287156e9f401d5395.camel@mediatek.com>
 <YxeR0RoiTdm8sWCJ@slm.duckdns.org>
 <02b8e7b3-941d-8bb9-cd0e-992738893ba3@redhat.com>
 <36a73401-7011-834a-7949-c65a2f66246c@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <36a73401-7011-834a-7949-c65a2f66246c@redhat.com>
Precedence: bulk

On Tue, Sep 06, 2022 at 04:40:03PM -0400, Waiman Long wrote:

I've not followed the earlier stuff due to being unreadable; just
reacting to this..

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 838623b68031..5d9ea1553ec0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2794,9 +2794,9 @@ static int __set_cpus_allowed_ptr_locked(struct
> task_struct *p,
> ??????????????? if (cpumask_equal(&p->cpus_mask, new_mask))
> ??????????????????????? goto out;
> 
> -?????????????? if (WARN_ON_ONCE(p == current &&
> -??????????????????????????????? is_migration_disabled(p) &&
> -??????????????????????????????? !cpumask_test_cpu(task_cpu(p), new_mask)))
> {
> +?????????????? if (is_migration_disabled(p) &&
> +?????????????????? !cpumask_test_cpu(task_cpu(p), new_mask)) {
> +?????????????????????? WARN_ON_ONCE(p == current);
> ??????????????????????? ret = -EBUSY;
> ??????????????????????? goto out;
> ??????????????? }
> @@ -2818,7 +2818,11 @@ static int __set_cpus_allowed_ptr_locked(struct
> task_struct *p,
> ??????? if (flags & SCA_USER)
> ??????????????? user_mask = clear_user_cpus_ptr(p);
> 
> -?????? ret = affine_move_task(rq, p, rf, dest_cpu, flags);
> +?????? if (!is_migration_disabled(p) || (flags & SCA_MIGRATE_ENABLE)) {
> +?????????????? ret = affine_move_task(rq, p, rf, dest_cpu, flags);
> +?????? } else {
> +?????????????? task_rq_unlock(rq, p, rf);
> +?????? }

This cannot be right. There might be previous set_cpus_allowed_ptr()
callers that are blocked and waiting for the task to land on a valid
CPU.