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

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Date:	Tue, 06 Apr 2010 19:20:34 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Darren Hart <dvhltc@...ibm.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, 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 04/06/2010 07:14 PM, Thomas Gleixner wrote:
>
>> IMO the best solution is to spin in userspace while the lock holder is
>> running, fall into the kernel when it is scheduled out.
>>      
> That's just not realistic as user space has no idea whether the lock
> holder is running or not and when it's scheduled out without a syscall :)
>    

The kernel could easily expose this information by writing into the 
thread's TLS area.

So:

- the kernel maintains a current_cpu field in a thread's tls
- lock() atomically writes a pointer to the current thread's current_cpu 
when acquiring
- the kernel writes an invalid value to current_cpu when switching out
- a contended lock() retrieves the current_cpu pointer, and spins as 
long as it is a valid cpu

-- 
error compiling committee.c: too many arguments to function

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