handle race to mkdir at startup

handle race to mkdir at startup

I ran into a problem when specifying -D dirname-that-doesnt-yet-exist.
Blktrace would fail, spewing the following messages:

[root@megadeth blktrace]# ./blktrace -d /dev/cciss/c0d1 -D ./2.6.30-rc2-cfq-local
Destination dir ./2.6.30-rc2-cfq-local/ can't be made: 17/File exists
Destination dir ./2.6.30-rc2-cfq-local/ can't be made: 17/File exists
Destination dir ./2.6.30-rc2-cfq-local/ can't be made: 17/File exists
Destination dir ./2.6.30-rc2-cfq-local/ can't be made: 17/File exists
Destination dir ./2.6.30-rc2-cfq-local/ can't be made: 17/File exists
FAILED to start thread on CPU 0: 1/Operation not permitted
FAILED to start thread on CPU 4: 1/Operation not permitted
FAILED to start thread on CPU 5: 1/Operation not permitted
FAILED to start thread on CPU 6: 1/Operation not permitted
FAILED to start thread on CPU 7: 1/Operation not permitted

I tracked it down to the fact that there is no synchronization between
threads when trying to create the output directory.  The fix is simple,
just allow the race to happen and detect it.  It's not really worth
putting in any extra synchronization.  It looks like no place else in
that startup path needs synchronization either.

This patch fixes the issue for me.  I tested it by running the very
command that caused me headaches 100% of the time before.  I also did a
chattr +i on the directory and verified that it would really fail in the
case where it couldn't create the directory.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>