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

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Message-ID: <AANLkTikayCJyYVo1MNpg1EN+88bSQNOu=NPW5s_j4shD@mail.gmail.com>
Date:	Thu, 24 Mar 2011 21:38:58 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Tejun Heo <tj@...nel.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()

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.

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.

                       Linus
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