linux-kernel - Re: [PATCH -v9][RFC] mutex: implement adaptive spinning

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Date:	Tue, 13 Jan 2009 17:39:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH -v9][RFC] mutex: implement adaptive spinning


* Peter Zijlstra <peterz@...radead.org> wrote:

> > Now you're forcing the slow-path on unlock. Maybe it was intentional, 
> > maybe it wasn't. Did you perhaps mean
> > 
> > 	if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
> > 
> > here? I thought we agreed it was safe, if only because it should be 
> > equivalent to just having done "mutex_trylock()" instead of a "real" 
> > lock sequence.
> 
> Yes, that was an 'accident' from -v8, yes we did think the cmpxchg was 
> good, however I did get some spurious lockups on -v7, and I only noticed 
> the thing after I'd done most of the testing, so I decided to let it be 
> for now.
> 
> Let me put the cmpxchg back in and see if this is all still good (only 
> 3*2*2 configs to test :-).

i saw sporadic lockups with -v7 too, so if you send a -v10 with Linus's 
sequence for the unlock it takes about an hour of testing to check whether 
it still occurs.

	Ingo
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