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

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Message-ID: <46382CA2.1020504@haxent.com.br>
Date:	Wed, 02 May 2007 03:16:02 -0300
From:	Davi Arnaut <davi@...ent.com.br>
To:	Eric Dumazet <dada1@...mosbay.com>
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

Eric Dumazet wrote:
> 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.
> 

Hmm, how about:

struct futex_event {
	union {
		void __user *addr;
		u64 padding;
	};
	union {
		int val;
		s64 val64;
	};
	/* whatever room is necessary for future improvements */
};

I haven't been keeping up with 64 bit or private futexes. What else
could probably go wrong?

--
Davi Arnaut

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