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

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Message-ID: <463839BD.4030008@haxent.com.br>
Date:	Wed, 02 May 2007 04:11:57 -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 :
>> 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?
> 
> Well, that's the point : This interface is like an ioctl() one : pretty bad if 
> not properly designed :)
> 
> You probably need to stick one field containing one command or version number, 
> something like that.
> 
> 
> struct futex_event {
> 	int     type;
> 	union {
> 		void __user *addr;
> 		u64 padding;
> 	};
> 	union {
> 		int val;
> 		s64 val64;
> 	};
> };
> 
> #define  FUTEX_EVENT_SHARED32  1
> #define  FUTEX_EVENT_SHARED64  2
> #define  FUTEX_EVENT_PRIVATE32 (128|1)
> #define  FUTEX_EVENT_PRIVATE64 (128|2)
> 

I'm changing the structure to:

struct futex_event {
	union {
		void __user *addr;
		u64 addr64;
	};
	union {
		int val;
		s64 val64;
	};
	union {
		s32 flags;
		s64 flags64;
	};
};

Plenty room for future FUTEX_WAIT growth ?

--
Davi Arnaut

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