linux-kernel - Re: [patch 1/3] rtmutex: Add missing deadlock check

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Date:	Tue, 13 May 2014 10:45:05 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Lai Jiangshan <laijs@...fujitsu.com>
cc:	LKML <linux-kernel@...r.kernel.org>, Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <darren@...art.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Clark Williams <williams@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Carlos ODonell <carlos@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch 1/3] rtmutex: Add missing deadlock check

Lai,

On Tue, 13 May 2014, Lai Jiangshan wrote:
> I think this patch is just a workaround, it is not the proper fix.
> you need a updated deadlock-check mechanism:
> 
> - (old) skip the check when top_waiter != task_top_pi_waiter(task)
> + (new) skip the check when top_waiter->prio > task->prio
> 
> 	/*
> 	 * Drop out, when the task has no waiters. Note,
> 	 * top_waiter can be NULL, when we are in the deboosting
> 	 * mode!
> 	 */
> 	if (top_waiter && (!task_has_pi_waiters(task) ||
> 			   top_waiter != task_top_pi_waiter(task)))
> 		goto out_unlock_pi;
> 
> 
> (also need to update the code in other places respectively)

Ok, I did not think it through fully and I want to have the rtmutex
tester working again so we can check for this without going through
futex hoops and loops. I had no time yet to look into that as I needed
to understand the futex issue which exposed it first.

> On 05/13/2014 04:45 AM, Thomas Gleixner wrote:
> >  	/*
> > +	 * Deadlock check for the following scenario:
> > +	 *
> > +	 * T holds lock L and has waiters
> > +	 * T locks L again, but does not end up as it's own top waiter
> 
> ABBA problem (TA TB TC TD are of the same priority)
> 
> TA holds lock LA, and try to lock LB which TC already has waited on
> TB holds lock LB, and try to lock LA which TD already has waited on
> 
> I think this check can't detect it IIUC.
> 
> > +	 *
> > +	 * So we would drop out at the next check without noticing.
> > +	 *
> > +	 * Note, we need to check for orig_waiter as it might be NULL
> > +	 * when deboosting!
> > +	 */
> > +	if (orig_waiter && orig_waiter->task == rt_mutex_owner(lock)) {
> 
> when non-first-loop, it is already checked.

Right, but we must check it for the first loop as well. And that check
was not there ever, so it's not your problem. I verified against a
kernel w/o your optimization.
 
> > +		ret = deadlock_detect ? -EDEADLK : 0;
> > +		goto out_unlock_pi;
> > +	}
> 
> I considered you blamed to me.
> I would feel better if you directly blamed to me.

I blamed you as well for not following up and updating the stuff you
broke.

Thanks,

	tglx
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