linux-kernel - Re: [PATCH 2/2] mutex: Apply adaptive spinning on mutex

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Message-ID: <20110325065300.GB1409@htj.dyndns.org>
Date:	Fri, 25 Mar 2011 07:53:00 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 2/2] mutex: Apply adaptive spinning on mutex_trylock()

Hello, Steven, Linus.

On Thu, Mar 24, 2011 at 09:38:58PM -0700, Linus Torvalds wrote:
> On Thu, Mar 24, 2011 at 8:39 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > But now, mutex_trylock(B) becomes a spinner too, and since the B's owner
> > is running (spinning on A) it will spin as well waiting for A's owner to
> > release it. Unfortunately, A's owner is also spinning waiting for B to
> > release it.
> >
> > If both A and B's owners are real time tasks, then boom! deadlock.
> 
> Hmm. I think you're right. And it looks pretty fundamental - I don't
> see any reasonable approach to avoid it.

Hmmm... I have an idea.  Will play with it a bit and post if it works
out okay.

> I think the RT issue is a red herring too - afaik, you can get a
> deadlock with two perfectly normal processes too. Of course, for
> non-RT tasks, any other process will eventually disturb the situation
> and you'd get kicked out due to need_resched(), but even that might be
> avoided for a long time if there are other CPU's - leading to tons of
> wasted CPU time.

Yeap, need_resched() currently is the only thing which limits the
duration of spinning when the owner continues to run.

Thanks.

-- 
tejun
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