Re: [PATCH] 2.6.16 - futex: small optimization (?)

From: Pierre PEIFFER
Date: Wed Mar 29 2006 - 08:15:20 EST


Ulrich Drepper a écrit :

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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