linux-kernel - second, bigger problem with private futexes

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Message-ID: <4651D628.1070206@redhat.com>
Date:	Mon, 21 May 2007 10:26:00 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>
Subject: second, bigger problem with private futexes

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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.

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*).

- --
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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