linux-kernel - Re: [patch 14/22] pollfs: pollable futex

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Date:	Wed, 2 May 2007 09:46:53 -0700
From:	"Ulrich Drepper" <drepper@...il.com>
To:	"Davi Arnaut" <davi@...ent.com.br>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Davide Libenzi" <davidel@...ilserver.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [patch 14/22] pollfs: pollable futex

On 5/2/07, Davi Arnaut <davi@...ent.com.br> wrote:
> It's quite easy to implement this scheme by write()ing the futexes all
> at once but that would break the one futex per fd association. For
> atomicity: if one of the futexes can't be queued, we would rollback
> (unqueue) the others.
>
> Sounds sane?

I don't know how you use "unqueue" in this context.  If a queued futex
is one which is /locked/ by te call, then yes, this is the semantics
needed.  Atomically locking a number of futexes means that if one of
the set cannot be locked all operations done to lock the others have
to be undone.  It's an all-or-nothing situation.

Locking is not as easy as you might think, though.  For non-PI futexes
there is deliberately no protocol in place describing what "locked"
means.  The locking operation has to be customizable.  This is what
the FUTEX_OP_* stuff is about.

And you wrote that currently each futex needs its own file descriptor.
 So this would have to be changed, too.
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