Re: [patch 14/22] pollfs: pollable futex

[Eric Dumazet]

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@xxxxxxxxxxxxx>
> > >
> > > ---
> > >  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)

...

Also, you should take care of alignements constraints (a 32bit user program 
might run on a 64bit kernel)

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