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From: Arnd Bergmann <arnd@arndb.de>
Date: Mon, 6 Jul 2009 17:05:50 +0200
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Subject: Re: [LTP] mmap syscall problem
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To: michal.simek@petalogix.com
Cc: LTP <ltp-list@lists.sourceforge.net>, Linux Kernel list <linux-kernel@vger.kernel.org>, Grant Likely <grant.likely@secretlab.ca>, Paul Mundt <lethal@linux-sh.org>, Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>, John Williams <john.williams@petalogix.com>

On Monday 06 July 2009, Michal Simek wrote:
> >       *p1 = 0xaa; *p2 = 0x55;
> >   
> I closed fd too.
>     close(fd);
> >       return *p1; /* returns 0xaa if broken, 0x55 if correct */
> > }

If you close the fd between the assignment and reading
from the pointer again, the test case becomes invalid because of
timing. Closing the fd before the '*p1 = 0xaa' should be fine,
but unnecessary.

I also realized that you might need to mark the pointers
as 'volatile' so that the compiler has to do the operations
in order.

> >   
> # ls -la existing-4k-file
> -rw-rw-r--    1 monstr   monstr       4096 Jul  6  2009 existing-4k-file
> 
> # ./test-arnd
> # echo $?
> 85

Ok, so inside a single task, this does not happen.

> # dd if=existing-4k-file of=/dev/console count=1 2>/dev/null
> U#
> 
> in file is first char U (0x55) which is IMO correct.

Right, though that was not part of the test, I'd expect this in
the file even if the return value was broken.

	Arnd <><

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