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

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Message-ID: <4651E1D4.6010705@cosmosbay.com>
Date:	Mon, 21 May 2007 20:15:48 +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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> 
> This one is a big problem:
> 
> If I understand the code correctly, a FUTEX_CMP_REQUEUE from a private
> futex will add the waiters to the other futex as a private futex.  And
> similarly for shared.
> 
> I.e., it is not possible to have one futex private and the other shared.
>  This is a huge problem.  The shared/private status of a conditional
> variable and the mutex used with it don't have to match.  But this is
> where FUTEX_CMP_REQUEUE is used.
> 
> If this is not changed (assuming I'm right with my analysis of the
> kernel code) this means mutexes and condvars will not be able to use
> private futexes.

Do you mean POSIX allowed to mix PROCESS_PRIVATE and PROCESS_SHARED condvar 
and mutexes ? Seems silly to me :(

> 
> What would be needed is an additional parameter for FUTEX_CMP_REQUEUE
> and FUTEX_CMP_REQUEUE_PI which specifies the state (shared/private) of
> the target futex.  The original futex' state is encoded in the command
> (the FUTEX_PRIVATE_FLAG ORed to FUTEX_CMP_REQUEUE*).
> 

Well, I guess it should be easy to add this if really necessary.


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