linux-kernel - Re: select_task_rq_fair: WARNING: at kernel/lockdep.c match_held

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Date:	Fri, 5 Aug 2011 18:27:55 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org
Subject: Re: select_task_rq_fair: WARNING: at kernel/lockdep.c match_held_lock

On (08/05/11 14:36), Peter Zijlstra wrote:
> The below is what I've come up with.
>

Hello,
I think that should work. Will test, anyway.

Thanks,
	Sergey
 
> ---
> Subject: lockdep: Fix wrong assumption in match_held_lock
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Fri Aug 05 14:26:17 CEST 2011
> 
> match_held_lock() was assuming it was being called on a lock class
> that had already seen usage. 
> 
> This condition was true for bug-free code using lockdep_assert_held(),
> since you're in fact holding the lock when calling it. However the
> assumption fails the moment you assume the assertion can fail, which
> is the whole point of having the assertion in the first place.
> 
> Anyway, now that there's more lockdep_is_held() users, notably
> __rcu_dereference_check(), its much easier to trigger this since we
> test for a number of locks and we only need to hold any one of them to
> be good.
> 
> Reported-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  kernel/lockdep.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/kernel/lockdep.c
> ===================================================================
> --- linux-2.6.orig/kernel/lockdep.c
> +++ linux-2.6/kernel/lockdep.c
> @@ -3111,7 +3111,13 @@ static int match_held_lock(struct held_l
>  		if (!class)
>  			class = look_up_lock_class(lock, 0);
>  
> -		if (DEBUG_LOCKS_WARN_ON(!class))
> +		/*
> +		 * If look_up_lock_class() failed to find a class, we're trying
> +		 * to test if we hold a lock that has never yet been acquired.
> +		 * Clearly if the lock hasn't been acquired _ever_, we're not
> +		 * holding it either, so report failure.
> +		 */
> +		if (!class)
>  			return 0;
>  
>  		if (DEBUG_LOCKS_WARN_ON(!hlock->nest_lock))
> 
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