linux-kernel - Re: [PATCH 1/7] lockdep: Print a nice description of an irq locking issue

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Date:	Thu, 21 Apr 2011 07:40:29 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/7] lockdep: Print a nice description of an irq
 locking issue

On Thu, 2011-04-21 at 15:02 +0800, Yong Zhang wrote:
> On Thu, Apr 21, 2011 at 9:41 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> >  ---
> >
> > The above is the case when the unsafe lock is taken while holding
> > a lock taken in irq context. But when a lock is taken that also
> > grabs a unsafe lock, the call chain is shown:
> >
> >  ---
> > other info that might help us debug this:
> >
> > Chain exists of:
> >  &rq->lock --> lockA --> lockC
> >
> >  Possible interrupt unsafe locking scenario:
> >
> >       CPU0                    CPU1
> >       ----                    ----
> >  lock(lockC);
> >                               local_irq_disable();
> >                               lock(&rq->lock);
> >                               lock(lockA);
> >  <Interrupt>
> >    lock(&rq->lock);
> >
> >  *** DEADLOCK ***
> 
> Or we could show this:
> Chain exists of:
> &rq->lock --> lockA --> lockC
> 
>  Possible interrupt unsafe locking scenario:
> 
>       CPU0                    CPU1                           CPU2
>       ----                    ----                                     ----
>  lock(lockC);
>                               local_irq_disable();
>                               lock(&rq->lock);            lock(lockA);
>                               lock(lockA);                   lock(lockC);
>  <Interrupt>
>    lock(&rq->lock);
> 
>  *** DEADLOCK ***


We could but I prefer not to ;) We have some chains that are 8 locks
deep. I really don't want to scatter that entirely across the screen.
Hence my "Chain exists.." statement, following an example that any
kernel developer can (with a little thinking) see is a possible
deadlock.

In fact, this code doesn't even look at the full chain, it only examines
3 locks in the chain, and lets the developer figure out the rest. I
could show the entire chain too.

Thanks,

-- Steve


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