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>

diff --git a/blktrace.c b/blktrace.c
index 656ab7a811ad8575f357c69d8d200cb7e982bd34..b4c919df18eec694f7e2e3e11220e0d4155b487a 100644 (file)
--- a/blktrace.c
+++ b/blktrace.c
@@ -1477,7 +1477,12 @@ static int fill_ofname(struct io_info *iop, int cpu)
                                iop->ofn, errno, strerror(errno));
                        return 1;
                }
-               if (mkdir(iop->ofn, 0755) < 0) {
+               /*
+                * There is no synchronization between multiple threads
+                * trying to create the directory at once.  It's harmless
+                * to let them try, so just detect the problem and move on.
+                */
+               if (mkdir(iop->ofn, 0755) < 0 && errno != EEXIST) {
                        fprintf(stderr,
                                "Destination dir %s can't be made: %d/%s\n",
                                iop->ofn, errno, strerror(errno));