From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH -v4][RFC]: mutex: implement adaptive spinning
Date: Wed, 7 Jan 2009 09:50:57 -0800 (PST)
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Cc: Peter Zijlstra <peterz@infradead.org>, paulmck@linux.vnet.ibm.com,
	Gregory Haskins <ghaskins@novell.com>,
	Ingo Molnar <mingo@elte.hu>, Matthew Wilcox <matthew@wil.cx>,
	Andi Kleen <andi@firstfloor.org>,
	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>,
	Steven Rostedt <rostedt@goodmis.org>,
	Nick Piggin <npiggin@suse.de>,
	Peter Morreale <pmorreale@novell.com>,
	Sven Dietrich <SDietrich@novell.com>
To: Chris Mason <chris.mason@oracle.com>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
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On Wed, 7 Jan 2009, Chris Mason wrote:
> 
> So far I haven't found any btrfs benchmarks where this is slower than
> mutexes without any spinning.  But, it isn't quite as fast as the btrfs
> spin.

Quite frankly, from our history with ext3 and other filesystems, using a 
mutex in the filesystem is generally the wrong thing to do anyway. 

Are you sure you can't just use a spinlock, and just release it over IO? 
The "have to do IO or extend the btree" case is usually pretty damn clear.

Because it really sounds like you're lock-limited, and you should just try 
to clean it up. A pure "just spinlock" in the hotpath is always going to 
be better.

		Linus