linux-kernel - Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Thu, 08 Jan 2009 18:23:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	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>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

On Thu, 2009-01-08 at 08:58 -0800, Linus Torvalds wrote:
> 
> Ok, I've gone through -v7, and I'm sure you're all shocked to hear it, but 
> I have no complaints. 

*cheer*, except I guess we need to figure out what goes bad for Chris.

> Except that you dropped all the good commit 
> commentary you had earlier ;)

Yeah, I've yet to add that back, will do.

> The patch looks pretty good (except for the big "#if 0" block in 
> mutex-debug.c that I hope gets fixed, but I can't even really claim that I 
> can be bothered), the locking looks fine (ie no locking at all), and the 
> numbers seem pretty convinving.
> 
> Oh, and I think the open-coded
> 
> 	atomic_cmpxchg(count, 1, 0) == 1
> 
> could possibly just be replaced with a simple __mutex_fastpath_trylock(). 
> I dunno.

__mutex_fastpath_trylock() isn't always that neat -- see
include/asm-generic/mutex-xchg.h -- and its a NOP on DEBUG_MUTEXES.

Note how I used old_val for the list_empty() thing as well, we could
possibly drop that extra condition though.

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