linux-kernel - Re: file offset corruption on 32-bit machines?

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Date:	Fri, 11 Apr 2008 14:24:34 +0200
From:	Bodo Eggert <7eggert@....de>
To:	Diego Calleja <diegocg@...il.com>, Jiri Kosina <jkosina@...e.cz>,
	Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.cz>,
	Meelis Roos <mroos@...ux.ee>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: file offset corruption on 32-bit machines?

Diego Calleja <diegocg@...il.com> wrote:

> El Thu, 10 Apr 2008 16:31:09 +0200 (CEST), Jiri Kosina <jkosina@...e.cz>
> escribió:
> 
>> I think this is worth fixing.
> 
> This question comes very often, and Linus even wrote a patch
> (http://lkml.org/lkml/2006/4/13/124 , http://lkml.org/lkml/2006/4/13/130)
> 
> But apparently there's no much interest in fixing it, because it would
> slow down some workloads...

AS far as I understand, the race is e.g.:

fpos := A:a, we want to make process/thread a read A:b or B:a without it
being a correct value in fpos. a!=b!=c, A!=B, A!=C.

a: read fpos.high (A:?)
b: write fpos (B:b)
a: read fpos.low (A:b)


If you change this to 

a: read fpos.high
a: read fpos.low
a: read fpos.high
a: read fpos.low

and compare the results, you need to

a: read fpos.high (A:?)
b: write fpos (B:b)
a: read fpos.low (A:b)
b: write fpos (A:c)
a: read fpos.high (A:b),(A:?)
b: write fpos (C:b)
a: read fpos.low (A:b),(A:b)

That would be winning three races in order to hit the bug. 


OTOH, writers MUST NOT be interrupted, because:

b: write fpos.high (B:a)
a: read fpos.high (B:?)
a: read fpos.low (B:a)
a: read fpos.high (B:a),(B:?)
a: read fpos.low (B:a),(B:a)
b: write fpos.low (B:b)


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