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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Michael Trimarchi <trimarchimichael@yahoo.it>
Subject: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
Date: Wed, 17 Sep 2008 00:11:21 +1000
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Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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On Tuesday 16 September 2008 23:57, Michael Trimarchi wrote:
> Hi,
>
>
>
> ----- Messaggio originale -----
>
> > Da: Nick Piggin <nickpiggin@yahoo.com.au>
> > A: Michael Trimarchi <trimarchimichael@yahoo.it>
> > Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>;
> > akpm@linux-foundation.org; linux-kernel@vger.kernel.org;
> > linux-fsdevel@vger.kernel.org Inviato: Marted́ 16 settembre 2008,
> > 14:54:21
> > Oggetto: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
> >
> > On Tuesday 16 September 2008 21:11, Michael Trimarchi wrote:
> > > Hi,
> > >
> > > > +        if (offset != file_pos_read(file))
> > > > +            file_pos_write(file, offset);
> > > >         retval = offset;
> > > >     }
> > >
> > > Is not more efficient here a compare xchg operation?
> >
> > The compiler should do it if it was.
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel"
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> > Please read the FAQ at  http://www.tux.org/lkml/
>
>  i was thinking about introducing a file_pos_update() not implemented using
> file_pos_read()/file_pos_write() and taking the seq_lock or
> disabling preemption only one time

AFAIK, the only way to do an atomic 64 bit store on 32-bit x86 is
"cmpxchg8b" (with lock prefix on SMP). That would be far slower
I'm sure.