From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 2/2] mutex: Apply adaptive spinning on mutex_trylock()
Date: Thu, 24 Mar 2011 21:38:58 -0700
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 <20110324094151.GE12038@htj.dyndns.org> <20110325033956.GB9313@home.goodmis.org>
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Cc: Tejun Heo <tj@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Chris Mason <chris.mason@oracle.com>,
	linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org
To: Steven Rostedt <rostedt@goodmis.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20110325033956.GB9313@home.goodmis.org>
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On Thu, Mar 24, 2011 at 8:39 PM, Steven Rostedt <rostedt@goodmis.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