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

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Date:	Wed, 02 May 2007 07:54:34 +0200
From:	Eric Dumazet <dada1@...mosbay.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

Davi Arnaut a écrit :
> Asynchronously wait for FUTEX_WAKE operation on a futex if it still contains
> a given value. There can be only one futex wait per file descriptor. However,
> it can be rearmed (possibly at a different address) anytime.
> 
> The pollable futex approach is far superior (send and receive events from
> userspace or kernel) to eventfd and fixes (supercedes) FUTEX_FD at the same time.
> 
> Building block for pollable semaphores and user-defined events.
> 
> Signed-off-by: Davi E. M. Arnaut <davi@...ent.com.br>
> 
> ---
>  fs/pollfs/Makefile |    1 
>  fs/pollfs/futex.c  |  154 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  init/Kconfig       |    7 ++
>  3 files changed, 162 insertions(+)
> 
> Index: linux-2.6/fs/pollfs/Makefile
> ===================================================================
> --- linux-2.6.orig/fs/pollfs/Makefile
> +++ linux-2.6/fs/pollfs/Makefile
> @@ -3,3 +3,4 @@ pollfs-y := file.o
>  
>  pollfs-$(CONFIG_POLLFS_SIGNAL) += signal.o
>  pollfs-$(CONFIG_POLLFS_TIMER) += timer.o
> +pollfs-$(CONFIG_POLLFS_FUTEX) += futex.o
> Index: linux-2.6/fs/pollfs/futex.c
> ===================================================================
> --- /dev/null
> +++ linux-2.6/fs/pollfs/futex.c
> @@ -0,0 +1,154 @@
> +/*
> + * pollable futex
> + *
> + * Copyright (C) 2007 Davi E. M. Arnaut
> + *
> + * Licensed under the GNU GPL. See the file COPYING for details.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/sched.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/err.h>
> +#include <linux/wait.h>
> +#include <linux/poll.h>
> +#include <linux/pollfs_fs.h>
> +#include <linux/futex.h>
> +
> +struct futex_event {
> +	union {
> +		void __user *addr;
> +		u64 padding;
> +	};
> +	int val;
> +};

Hum... Here we might have a problem with 64 bit futexes, or private futexes

So I believe this interface is not well defined and not expandable: in case of 
future additions to futexes, an old application compiled with an old pollable 
futex_event type might fail.



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