Messages in this thread | | | From | Bodo Eggert <> | Subject | Re: file offset corruption on 32-bit machines? | Date | Fri, 11 Apr 2008 14:24:34 +0200 |
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Diego Calleja <diegocg@gmail.com> wrote:
> El Thu, 10 Apr 2008 16:31:09 +0200 (CEST), Jiri Kosina <jkosina@suse.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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