Message-ID: <442A8933.6090408@bull.net>
Date: Wed, 29 Mar 2006 15:18:43 +0200
From: Pierre PEIFFER <pierre.peiffer@bull.net>
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To: Ulrich Drepper <drepper@gmail.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       jakub@redhat.com
Subject: Re: [PATCH] 2.6.16 - futex: small optimization (?)
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Ulrich Drepper a ?crit :
> 
> There are no such situations anymore in an optimal userlevel
> implementation.  The last problem (in pthread_cond_signal) was fixed
> by the addition of FUTEX_WAKE_OP.  The userlevel code you're looking
> at is simply not optimized for the modern kernels.
> 

I think there is a misunderstanding here.

FUTEX_WAKE_OP is implemented to handle simultaneously more than one 
futex in some specific situations (such as pthread_cond_signal).

The scenario I've described occurred in futex_wake, futex_wake_op and 
futex_requeue and is _independent_ of the userlevel code.

All these functions call wake_futex, and then wake_up_all, with the 
futex_hash_bucket lock still held.

If the woken thread is immediately scheduled (in wake_up_all), and only 
in this case (because of a higher priority, etc), it will try to take 
this lock too (because of the "if (lock_ptr != 0)" statement in 
unqueue_me), causing two task-switches to take this lock for nothing.

Otherwise, it will not: lock_ptr is set to NULL just after the 
wake_up_all call)

This scenario happens at least in pthread_cond_signal, 
pthread_cond_broadcast and probably all pthread_*_unlock functions.

The patch I've proposed should, at least in theory, solve this. But I'm 
not sure of the correctness...

-- 
Pierre P.
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