linux-kernel - Re: second, bigger problem with private futexes

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Message-ID: <4651F93C.2020806@cosmosbay.com>
Date:	Mon, 21 May 2007 21:55:40 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Ulrich Drepper <drepper@...hat.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: second, bigger problem with private futexes

Ulrich Drepper a écrit :
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> 
> Eric Dumazet wrote:
>> 3) if condvar is PRIVATE and mutex is SHARED, a FUTEX_WAKE_PRIVATE
>> should be done. (and loose the REQUEUE optim)
>> Yes we could add a special futex primitive for this special case. But I
>> cannot see how a program could use such a construct.
> 
> Very easily: mutexes are often created independently from the condvars
> and they are used for many things.  Maybe a program is even creating all
> mutexes as shared to be ready for all situations.  Normally doing this
> is no big problem, the performance penalties are minimal.
> 
> 
>> 4) if condvar is SHARED and mutex is private, we have a *problem*,
>> because the process doing the broadcast() can be in another mm. So a
>> requeue is not possible at all.
> 
> It is if we can specify the owner of the mutex.  I.e., the PID.

well, then we have a refcounting issue on pid , or problem in pid reuse.

> 
> But yes, this case is extremely ugly.
> 
> 
> The problem is that all these cases worked nice so far.  They all had
> the same good performance.  Now we are severely penalizing code which
> mismatches condvar and mutex shared attributes.  There is a good reason
> why we introduced FUTEX_CMP_REQUEUE, the benefits in certain programs is
> huge.

This analysis seems unfair to me, after a quick reading of glibc code.

Right now, glibc cannot use FUTEX_CMP_REQUEUE if condvar is pshared.

/* Do not use requeue for pshared condvars.  */
if (cond->__data.__mutex == (void *) ~0l)
	goto wake_all;

So how introducing private futexes is penalizing this case ?

Fact is that if condvar is pshared, we *cannot* use CMP_REQUEUE since threads 
doing the broadcast() can be in a separate process and virtual address of 
mutex of waiting threads could point to unrelated memory.

Thanks

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