linux-kernel - Re: [PATCH 2/4] rtmutex: add BUG_ON if a task attempts to block on two locks

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Date:	Fri, 09 Jul 2010 20:30:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	John Kacur <jkacur@...hat.com>, Mike Galbraith <efault@....de>,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 2/4] rtmutex: add BUG_ON if a task attempts to block on
 two locks

On Fri, 2010-07-09 at 15:32 -0700, Darren Hart wrote:
> rtmutex proxy locking complicates the logic a bit and opens up
> the possibility for a task to wake and attempt to take another
> sleeping lock without knowing it has been enqueued on another
> lock already. Add a BUG_ON to catch this scenario early.
> 
> Signed-off-by: Darren Hart <dvhltc@...ibm.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Cc: John Kacur <jkacur@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mike Galbraith <efault@....de>
> ---
>  kernel/rtmutex.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
> index baac7d9..22f9d18 100644
> --- a/kernel/rtmutex.c
> +++ b/kernel/rtmutex.c
> @@ -459,6 +459,9 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
>  		top_waiter = rt_mutex_top_waiter(lock);
>  	plist_add(&waiter->list_entry, &lock->wait_list);
>  
> +	/* Tasks can only block on one lock at a time. */
> +	BUG_ON(task->pi_blocked_on != NULL);

WARN_ON may be better. Since it may not cause a system crash or other
huge bug if it is not true.

-- Steve

> +
>  	task->pi_blocked_on = waiter;
>  
>  	raw_spin_unlock(&task->pi_lock);


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