linux-kernel - Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

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Message-ID: <20100406145128.6324ac9a@lxorguk.ukuu.org.uk>
Date:	Tue, 6 Apr 2010 14:51:28 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.com>, Darren Hart <dvhltc@...ibm.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Peter W. Morreale" <pmorreale@...ell.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Chris Mason <chris.mason@...cle.com>,
	John Cooper <john.cooper@...rd-harmonic.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
 spinning

On Tue, 06 Apr 2010 15:35:31 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2010-04-06 at 16:28 +0300, Avi Kivity wrote:
> > 
> > Yes, but that's the best case for spinning.  You could simply use a 
> > userspace spinlock in this case. 
> 
> Userspace spinlocks are evil.. they should _never_ be used.

Thats a gross and inaccurate simplification. For the case Avi is talking
about spinning in userspace makes sense in a lot of environments. Once
you've got one thread pinned per cpu (or gang scheduling >-) ) there are
various environments where it makes complete and utter sense.
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