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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>,
	kernel-janitors@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/rtmutex: Fix ww_mutex deadlock check
Date: Mon, 6 Sep 2021 18:51:49 +0200	[thread overview]
Message-ID: <20210906165149.4b6ghzxnoqfm6ram@linutronix.de> (raw)
In-Reply-To: <YS9La56fHMiCCo75@hirez.programming.kicks-ass.net>

On 2021-09-01 11:44:11 [+0200], Peter Zijlstra wrote:
> Subject: locking/rtmutex: Fix ww_mutex deadlock check
> 
> Dan reported that rt_mutex_adjust_prio_chain() can be called with
> .orig_waiter == NULL however commit a055fcc132d4 ("locking/rtmutex:
> Return success on deadlock for ww_mutex waiters") unconditionally
> dereferences it.
> 
> Since both call-sites that have .orig_waiter == NULL don't care for the
> return value, simply disable the deadlock squash by adding the NULL
> check.
> 
> Notably, both callers use the deadlock condition as a termination
> condition for the iteration; once detected, we're sure (de)boosting is
> done. Arguably [3] would be a more natural termination point, but I'm
> not sure adding a third deadlock detection state would improve the code.
> 
> Fixes: a055fcc132d4 ("locking/rtmutex: Return success on deadlock for ww_mutex waiters")
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

It sounds reasonable and I don't see any fallout in the testsuite so

  Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

> ---
>  kernel/locking/rtmutex.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 8eabdc79602b..6bb116c559b4 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -753,7 +753,7 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
>  		 * other configuration and we fail to report; also, see
>  		 * lockdep.
>  		 */
> -		if (IS_ENABLED(CONFIG_PREEMPT_RT) && orig_waiter->ww_ctx)
> +		if (IS_ENABLED(CONFIG_PREEMPT_RT) && orig_waiter && orig_waiter->ww_ctx)
>  			ret = 0;
>  
>  		raw_spin_unlock(&lock->wait_lock);

Sebastian

  reply	other threads:[~2021-09-06 16:51 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-31  8:21 [bug report] locking/rtmutex: Return success on deadlock for ww_mutex waiters Dan Carpenter
2021-09-01  8:09 ` Peter Zijlstra
2021-09-01  9:44   ` [PATCH] locking/rtmutex: Fix ww_mutex deadlock check Peter Zijlstra
2021-09-06 16:51     ` Sebastian Andrzej Siewior [this message]
2021-09-09  8:34     ` [tip: locking/urgent] " tip-bot2 for Peter Zijlstra

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