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

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Date:	Tue, 13 Jan 2009 08:16:19 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.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, 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.

			Linus
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