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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Sun, 1 Jan 2023 01:20:01 -0500
Message-ID: <CAEXW_YQyieW+w-VJ48N5kMkQo-RzJhFXD2kBR91B0KqhVFJNbw@mail.gmail.com>
Subject: Re: [PATCH] torture: Fix hang during kthread shutdown phase
To:     linux-kernel@vger.kernel.org
Cc:     Paul McKenney <paulmck@kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>, stable@vger.kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Josh Triplett <josh@joshtriplett.org>
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On Sun, Jan 1, 2023 at 1:16 AM Joel Fernandes (Google)
<joel@joelfernandes.org> wrote:
>
> During shutdown of rcutorture, the shutdown thread in
> rcu_torture_cleanup() calls torture_cleanup_begin() which sets fullstop
> to FULLSTOP_RMMOD. This is enough to cause the rcutorture threads for
> readers and fakewriters to breakout of their main while loop and start
> shutting down.
>
> Once out of their main loop, they then call torture_kthread_stopping()
> which in turn waits for kthread_stop() to be called, however
> rcu_torture_cleanup() has not even called kthread_stop() on those
> threads yet, it does that a bit later.  However, before it gets a chance
> to do so, torture_kthread_stopping() calls
> schedule_timeout_interruptible(1) in a tight loop. Tracing confirmed
> this makes the timer softirq constantly execute timer callbacks, while
> never returning back to the softirq exit path and is essentially "locked
> up" because of that. If the softirq preempts the shutdown thread,
> kthread_stop() may never be called.
>
> This commit improves the situation dramatically, by increasing timeout
> passed to schedule_timeout_interruptible() 1/20th of a second. This
> causes the timer softirq to not lock up a CPU and everything works fine.
> Testing has shown 100 runs of TREE07 passing reliably, which was not the
> case before because of RCU stalls.
>
> Cc: Paul McKenney <paulmck@kernel.org>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Zhouyi Zhou <zhouzhouyi@gmail.com>
> Cc: <stable@vger.kernel.org> # 6.0.x

Question for stable maintainers:
This patch is for mainline and 6.0 stable. However, it should also go
to 6.1 stable. How do we tag it to do that? I did not know how to tag
2 stable versions. I guess the above implies > 6.0 ?

Thanks,

 - Joel