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

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Message-ID: <20100406175311.11e5d3a8@lxorguk.ukuu.org.uk>
Date:	Tue, 6 Apr 2010 17:53:11 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Avi Kivity <avi@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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

> That works for the uncontended case.  For the contended case, the waiter 
> and the owner have to go into the kernel and back out to transfer 
> ownership.

If you insist on doing it that way yes, but knowing the lock owner is
likely to be away for a while also lets you do things like punt work
either by picking another work package in the meantime, or by queueing
the work you can't do on a list the pre-empted lock owner will review
before dropping the lock.

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