linux-kernel - Re: [PATCH v3] sched/deadline: Fix race in push_dl

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Message-ID: <Z_YGW8IrRQfhfdM8@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 9 Apr 2025 07:32:11 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Harshit Agarwal <harshit@...anix.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3] sched/deadline: Fix race in push_dl_task

Hello,

On 08/04/25 04:50, Harshit Agarwal wrote:
> When a CPU chooses to call push_dl_task and picks a task to push to
> another CPU's runqueue then it will call find_lock_later_rq method
> which would take a double lock on both CPUs' runqueues. If one of the
> locks aren't readily available, it may lead to dropping the current
> runqueue lock and reacquiring both the locks at once. During this window
> it is possible that the task is already migrated and is running on some
> other CPU. These cases are already handled. However, if the task is
> migrated and has already been executed and another CPU is now trying to
> wake it up (ttwu) such that it is queued again on the runqeue
> (on_rq is 1) and also if the task was run by the same CPU, then the
> current checks will pass even though the task was migrated out and is no
> longer in the pushable tasks list.
> Please go through the original rt change for more details on the issue.
> 
> To fix this, after the lock is obtained inside the find_lock_later_rq,
> it ensures that the task is still at the head of pushable tasks list.
> Also removed some checks that are no longer needed with the addition of
> this new check.
> However, the new check of pushable tasks list only applies when
> find_lock_later_rq is called by push_dl_task. For the other caller i.e.
> dl_task_offline_migration, existing checks are used.
> 
> Signed-off-by: Harshit Agarwal <harshit@...anix.com>
> Cc: stable@...r.kernel.org
> ---

The new version looks good to me.

Some final minor touches to changelog/comment might still be required,
but Peter maybe you can do it if/when picking up the change?

Anyway,

Acked-by: Juri Lelli <juri.lelli@...hat.com>

Thanks!
Juri