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

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Message-ID: <20080410152846.GG6725@atrey.karlin.mff.cuni.cz>
Date:	Thu, 10 Apr 2008 17:28:46 +0200
From:	Jan Kara <jack@...e.cz>
To:	Martin Mares <mj@....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?

> Martin Mares wrote:
> > > [*] file_pos_{read,write} (fs/read_write.c) are not called under
> > > lock (in sys_read, sys_write, ...), so even if f_pos is written
> > > atomically, you will be able to get races when accessing shared
> > > descriptor from different threads.
> > 
> > There are however cases when such behavior is perfectly valid: For example
> > you can have a file of records of a fixed size, whose order does not matter.
> > Then multiple processes can produce the records in parallel, sharing
> > a single fd.
> 
> A rather more common thing:
> 
> Does this problem apply when appending lines or records to a log file,
> with or without O_APPEND?
  O_APPEND works correctly in all cases (it ignores f_pos in the
descriptor). Without O_APPEND you can hit the race (but I'd like to see
a sensible use case of this ;).

> Also, can this problem affect programs doing concurrent reads/writes
> using pread/pwrite (or the AIO equivalents)?
  As Matthew said, pread/pwrite are safe, parallel read can hit the race,
write was described above...

									Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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