Message-ID: <4651E1D4.6010705@cosmosbay.com>
Date: Mon, 21 May 2007 20:15:48 +0200
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Ulrich Drepper <drepper@redhat.com>
CC: Linux Kernel <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: second, bigger problem with private futexes
References: <4651D628.1070206@redhat.com>
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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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