linux-kernel - Re: [PATCH 2/2] block: remove retry loop in ioc_release

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Date:   Tue, 23 Jun 2020 16:30:26 +0200
From:   Daniel Wagner <dwagner@...e.de>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] block: remove retry loop in ioc_release_fn()

On Fri, Jun 19, 2020 at 05:23:18PM +0206, John Ogness wrote:
> The reverse-order double lock dance in ioc_release_fn() is using a
> retry loop. This is a problem on PREEMPT_RT because it could preempt
> the task that would release q->queue_lock and thus live lock in the
> retry loop.
> 
> RCU is already managing the freeing of the request queue and icq. If
> the trylock fails, use RCU to guarantee that the request queue and
> icq are not freed and re-acquire the locks in the correct order,
> allowing forward progress.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Again, after starring on it for while and reading up and down,
I'd say, it looks good. Also a quick test run with blktests and
lockdep enabled didn't produce any warnings.

Reviewed-by: Daniel Wagner <dwagner@...e.de>