linux-kernel - Re: [PATCH][RFC]: mutex: adaptive spin

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Date:	Tue, 6 Jan 2009 18:02:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH][RFC]: mutex: adaptive spin


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, 6 Jan 2009, Linus Torvalds wrote:
> > 
> > Right now, if some process deadlocks on a mutex, we get hung process, 
> > but with a nice backtrace and hopefully other things (that don't need 
> > that lock) still continue to work.
> 
> Clarification: the "nice backtrace" we only get with something like 
> sysrq-W, of course. We don't get a backtrace _automatically_, but with 
> an otherwise live machine, there's a better chance that people do get 
> wchan or other info. IOW, it's at least a fairly debuggable situation.

btw., the softlockup watchdog detects non-progressing uninterruptible 
tasks (regardless of whether they locked up due to mutexes or any other 
reason).

This does occasionally help in debugging deadlocks:

  http://marc.info/?l=linux-mm&m=122889587725061&w=2

but it would indeed be also good to have the most common self-deadlock 
case checked unconditionally in the mutex slowpath.

	Ingo
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