From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [GIT PULL] adaptive spinning mutexes
Date: Thu, 15 Jan 2009 16:53:51 -0800
Message-ID: <20090116005351.GD6763@linux.vnet.ibm.com>
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Cc: Ingo Molnar <mingo@elte.hu>, Matthew Wilcox <matthew@wil.cx>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Gregory Haskins <ghaskins@novell.com>,
	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>,
	Johannes Weiner <hannes@cmpxchg.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
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On Thu, Jan 15, 2009 at 10:16:53AM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 15 Jan 2009, Ingo Molnar wrote:
> > 
> > btw., i think spin-mutexes have a design advantage here: in a lot of code 
> > areas it's quite difficult to use spinlocks - cannot allocate memory, 
> > cannot call any code that can sporadically block (but does not _normally_ 
> > block), etc.
> > 
> > With mutexes those atomicity constraints go away - and the performance 
> > profile should now be quite close to that of spinlocks as well.
> 
> Umm. Except if you wrote the code nicely and used spinlocks, you wouldn't 
> hold the lock over all those unnecessary and complex operations.
> 
> IOW, if you do pre-allocation instead of holding a lock over the 
> allocation, you win. So yes, spin-mutexes makes it easier to write the 
> code, but it also makes it easier to just plain be lazy.

In infrequently invoked code such as some error handling, lazy/simple
can be a big win.

							Thanx, Paul