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