linux-kernel - Re: [PATCH 001 of 2] Fix read/truncate race.

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Date:	Tue, 12 Jun 2007 10:16:22 +1000
From:	Neil Brown <neilb@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 001 of 2] Fix read/truncate race.

On Thursday June 7, akpm@...ux-foundation.org wrote:
> On Fri, 8 Jun 2007 12:48:48 +1000 Neil Brown <neilb@...e.de> wrote:
> 
> > The following patch will remove the extra seqlock except when we
> > actually need it and remove the extra arithmetic - but I haven't
> > tested it or reviewed it properly.  I can do that if you think it is
> > the right direction.
> 
> Yes, the optimisation is valid and looks useful.
> 
> >  ./mm/filemap.c |   34 ++++++++++++++++++++++++++++------
> 
> It didn't apply - your tree seems different from mine.

Odd.  I had no other changes to that file in my tree.  I'll wait until
the next -mm(?).
It's just as well really, the patch was buggy - didn't even compile
and (as I said) totally untested.  I'll get you a tested patch after I
can rebase.

> 
> > +		 *
> > +		 * NOTE: This access of inode->i_size is not protected
> > +		 *  and if there is a concurrent update on a 32bit machine,
> > +		 *  it could return the wrong value.  This could only be a problem
> > +		 *  if i_size has actually changed to a smaller value before the
> > +		 *  page became uptodate, and at this point it still has a smaller
> > +		 *  value, but due to a race while reading, it appears unchanged.
> > +		 *  The chances of this happening are so small and the consequence
> > +		 *  sufficiently minor, that the cost of the seqlock seems
> > +		 *  not to be justified.
> 
> please consider incorporating scripts/checkpatch.pl into your patch
> preparation toolchain.

Done... Any reason that it isn't executable (chmod +x)?

NeilBrown
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