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[dm-devel] [patch 06/24] dm: fix find_device race



From: Alasdair G Kergon <agk redhat com>

There is a race between dev_create() and find_device().

If the mdptr has not yet been stored against a device, find_device() needs to
behave as though no device was found.  It already returns NULL, but there is a
dm_put() missing: it must drop the reference dm_get_md() took.

The bug was introduced by dm-fix-mapped-device-ref-counting.patch.

It manifests itself if another dm ioctl attempts to reference a newly-created
device while the device creation ioctl is still running.  The consequence is
that the device cannot be removed until the machine is rebooted.  Certain udev
configurations can lead to this happening.

Signed-off-by: Alasdair G Kergon <agk redhat com>
Cc: <dm-devel redhat com>
Signed-off-by: Andrew Morton <akpm osdl org>
---

 drivers/md/dm-ioctl.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff -puN drivers/md/dm-ioctl.c~dm-fix-find_device-race drivers/md/dm-ioctl.c
--- a/drivers/md/dm-ioctl.c~dm-fix-find_device-race
+++ a/drivers/md/dm-ioctl.c
@@ -606,9 +606,14 @@ static struct hash_cell *__find_device_h
 		return __get_name_cell(param->name);
 
 	md = dm_get_md(huge_decode_dev(param->dev));
-	if (md)
-		mdptr = dm_get_mdptr(md);
+	if (!md)
+		goto out;
 
+	mdptr = dm_get_mdptr(md);
+	if (!mdptr)
+		dm_put(md);
+
+out:
 	return mdptr;
 }
 
_


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