linux-kernel - Re: [PATCH 2/3] task: RCU protect tasks on the runqueue

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Message-ID: <20190904142202.GA20391@lenoir>
Date:   Wed, 4 Sep 2019 16:22:03 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 2/3] task: RCU protect tasks on the runqueue

On Mon, Sep 02, 2019 at 11:52:01PM -0500, Eric W. Biederman wrote:
> 
> In the ordinary case today the rcu grace period of a task comes when a
> task is reaped, well after the task has left the runqueue.  This
> change guarantees that the rcu grace period always happens after a
> task has left the runqueue.  As this is something that usaually happens
> today I do not expect any code correctness problems with this change.
> At most I anticipate timing challenges.

What do you consider as the reaping point here? If this is the call to
release_task(), it can happen way before the task forever leaves the runqueue.

Let alone the RCU call to delayed_put_task_struct() can happen way before
the target leaves the runqueue, either after autoreap or normal reaping.