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Date: Tue, 6 Apr 2010 14:51:28 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Avi Kivity <avi@redhat.com>, Darren Hart <dvhltc@us.ibm.com>,
       linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
       Ingo Molnar <mingo@elte.hu>, Eric Dumazet <eric.dumazet@gmail.com>,
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       Rik van Riel <riel@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
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       Sven-Thorsten Dietrich <sdietrich@novell.com>,
       Chris Mason <chris.mason@oracle.com>,
       John Cooper <john.cooper@third-harmonic.com>,
       Chris Wright <chrisw@sous-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
 spinning
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On Tue, 06 Apr 2010 15:35:31 +0200
Peter Zijlstra <peterz@infradead.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.