Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch

Nick Piggin
2008-10-10T02:25:35Z

On Friday 10 October 2008 08:51, dcg wrote:
> El Wed, 08 Oct 2008 08:51:51 +0200, Peter Zijlstra <a.p.zijlstra@chello.nl>
escribió:
> > either dup() the fd or open() the file twice. There is absolutely no
> > valid reason to have two threads read from the same fd without
> > synchronising their access to it - never.
>
> In case this is the final consensus, I think that a topic that is brought
> to the list every few months and even generates (aparently not neccesary)
> patches is a hint that there should be somewhere a commentary (*) like
> this:
>
> (*) I don't know if what I wrote is 100% correct.
>
>
> Signed-off-by: Diego Calleja García <diegocg@gmail.com>
>
> Index: 2.6/include/linux/fs.h
> ===================================================================
> --- 2.6.orig/include/linux/fs.h 2008-10-09 00:06:50.000000000 +0200
> +++ 2.6/include/linux/fs.h 2008-10-09 00:29:03.000000000 +0200
> @@ -821,6 +821,18 @@
> atomic_long_t f_count;
> unsigned int f_flags;
> mode_t f_mode;
> + /*
> + * Linux does NOT guarantee atomic reading/writing to file->f_pos in
> + * multithread apps running in 32 bit machines. There're several
> + * reasons for this behaviour:

Note that I don't think we'd want to explicitly guarantee that it is atomic
on 64-bit machines either. It does happen to be, but I don't think we want
anybody to rely on that...