From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH -v9][RFC] mutex: implement adaptive spinning
Date: Tue, 13 Jan 2009 08:16:19 -0800 (PST)
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Cc: Ingo Molnar <mingo@elte.hu>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@novell.com>,
	Matthew Wilcox <matthew@wil.cx>,
	Andi Kleen <andi@firstfloor.org>,
	Chris Mason <chris.mason@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nick Piggin <npiggin@suse.de>,
	Peter Morreale <pmorreale@novell.com>,
	Sven Dietrich <SDietrich@novell.com>,
	Dmitry Adamushko <dmitry.adamushko@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
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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