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

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Date:	Wed, 16 Apr 2008 10:40:57 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Bryan Henderson <hbryan@...ibm.com>
Cc:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>,
	Bodo Eggert <7eggert@....de>,
	Diego Calleja <diegocg@...il.com>, Jan Kara <jack@...e.cz>,
	Jiri Kosina <jkosina@...e.cz>, linux-fsdevel@...r.kernel.org,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>, Meelis Roos <mroos@...ux.ee>,
	Pavel Machek <pavel@....cz>
Subject: Re: file offset corruption on 32-bit machines?

Bryan Henderson wrote:
> The easiest way to imagine a program not doing locking and being useful 
> anyway (as long as the kernel is thread-safe) is to use the same arguments 
> you use for the kernel doing it: there's a higher level user responsible 
> for locking.  The code in question doesn't guarantee that user writes all 
> its stuff to the right place, but at least it guarantees that that user's 
> lack of locking doesn't screw some other user of the file.  It does that 
> by ensuring it never seeks to a place the user doesn't own and that no two 
> separate users ever access the file at the same time.
> 
> I'd even like to accomodate the poor user trying to debug the broken 
> locking in his application.  He sees the file getting corrupted and 
> immediately thinks, "what if my thread serialization isn't working right?" 
>  But he notices that the corruption isn't consistent with that hypothesis. 
>  He knows he was working with only the beginning and the end of the file 
> and the corruption happened in the middle.  So he wastes a week 
> considering other hypotheses, including a kernel bug, until someone points 
> out a paragraph in the lseek() man page that says contrary to all Unix 
> convention, that particular function and system call is not thread-safe, 
> and it doesn't necessarily seek to the place mentioned in its argument.

I think that argument is the strongest yet.  Wasted debugging time due
to totally surprising and hardly justifiable kernel behaviour.  Strace
/ GDB on the application shows a trace which doesn't relate at all to
the unexpected file changes.

There is also POSIX specification:

  http://www.opengroup.org/onlinepubs/000095399/functions/xsh_chap02_09.html

  "All functions defined by this volume of IEEE Std 1003.1-2001 shall
be thread-safe, except that the following functions need not be
thread-safe."

  [List which does not include lseek(), therefore lseek() shall be
  thread-safe.  Same for read() and write().]

Docs for HP-UX and AIX say the same as POSIX about thread-safety.

-- Jamie
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