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