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

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Date:	Tue, 13 Jan 2009 17:21:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"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

On Tue, 2009-01-13 at 08:16 -0800, Linus Torvalds wrote:
> 
> On Tue, 13 Jan 2009, Peter Zijlstra wrote:
> > 
> > Change mutex contention behaviour such that it will sometimes busy wait on
> > acquisition - moving its behaviour closer to that of spinlocks.
> 
> Okey, dokey. Looks reasonable, but I wonder if this part came from v8 and 
> wasn't intentional:
> 
> > +		if (atomic_xchg(&lock->count, -1) == 1) {
> > +			lock_acquired(&lock->dep_map, ip);
> > +			mutex_set_owner(lock);
> > +			preempt_enable();
> > +			return 0;
> > +		}
> 
> 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 :-).
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