linux-kernel - Re: [PATCH RT] locking/rtmutex: don't drop the wait

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Message-ID: <20170921123105.5462f21d@gandalf.local.home>
Date:   Thu, 21 Sep 2017 12:31:05 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de
Subject: Re: [PATCH RT] locking/rtmutex: don't drop the wait_lock twice

On Thu, 21 Sep 2017 17:48:43 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> Since the futex rework, __rt_mutex_start_proxy_lock() does no longer
> acquire the wait_lock so it must not drop it. Otherwise the lock is not
> only unlocked twice but also the preemption counter is underflown.
> 
> Cc: rt-stable@...r.kernel.org
> Reported-by: Gusenleitner Klaus <gus@...a.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/locking/rtmutex.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index f03876322d4a..79f49d73e4d0 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -2281,7 +2281,6 @@ int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
>  	raw_spin_lock(&task->pi_lock);
>  	if (task->pi_blocked_on) {
>  		raw_spin_unlock(&task->pi_lock);
> -		raw_spin_unlock_irq(&lock->wait_lock);

Hmm, before this patch, irqs are enabled when returning with -EAGAIN.
But now they are not. Should that be:

		raw_spin_unlock_irq(&taks->pi_lock);

or is there something that changes this?

-- Steve


>  		return -EAGAIN;
>  	}
>  	task->pi_blocked_on = PI_REQUEUE_INPROGRESS;