linux-kernel - Re: [PATCH 1/1] rtmutex: Handle when top lock owner changes

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Date:	Wed, 4 Jun 2014 15:07:51 -0500
From:	"Brad Mouring" <bmouring@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Brad Mouring <bmouring@...com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 1/1] rtmutex: Handle when top lock owner changes

On Wed, Jun 04, 2014 at 09:53:16PM +0200, Thomas Gleixner wrote:
> On Wed, 4 Jun 2014, Brad Mouring wrote:
> > On Wed, Jun 04, 2014 at 08:02:16PM +0200, Thomas Gleixner wrote:
> > > I'll fixup the check so it wont break the real deadlock case and queue
> > > it.
> > 
> > How would the change break the real deadlock case?
> 
> > >  	/* Deadlock detection */
> > >  	if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
> > > +		/*
> > > +		 * If the prio chain has changed out from under us, set the task
> > > +		 * to the current owner of the lock in the current waiter and
> > > +		 * continue walking the prio chain
> > > +		 */
> > > +		if (rt_mutex_owner(lock) && rt_mutex_owner(lock) != task) {
> 
> No, sorry. That's wrong.
> 
> Your change wreckages the rt_mutex_owner(lock) == top_task test
> simply because in that case:
> 
>    (rt_mutex_owner(lock) && rt_mutex_owner(lock) != task)
> 
> evaluates to true.

Ah. Yeah. I haven't tested this but it seems sane to me.

> 
> So we want this:
> 
> Index: tip/kernel/locking/rtmutex.c
> ===================================================================
> --- tip.orig/kernel/locking/rtmutex.c
> +++ tip/kernel/locking/rtmutex.c
> @@ -375,6 +375,26 @@ static int rt_mutex_adjust_prio_chain(st
>  	 * walk, we detected a deadlock.
>  	 */
>  	if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
> +		/*
> +		 * If the prio chain has changed out from under us, set the task
> +		 * to the current owner of the lock in the current waiter and
> +		 * continue walking the prio chain
> +		 */
> +		if (rt_mutex_owner(lock) && rt_mutex_owner(lock) != task &&
> +		    rt_mutex_owner(lock) != top_task) {
> +			/* Release the old owner */
> +			raw_spin_unlock_irqrestore(&task->pi_lock, flags);
> +			put_task_struct(task);
> +
> +			/* Move to the new owner */
> +			task = rt_mutex_owner(lock);
> +			get_task_struct(task);
> +
> +			/* Let's try this again */
> +			raw_spin_unlock(&lock->wait_lock);
> +			goto retry;
> +		}
> +
>  		debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
>  		raw_spin_unlock(&lock->wait_lock);
>  		ret = deadlock_detect ? -EDEADLK : 0;
> --
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