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Subject[PATCH 3.16 074/136] dm: fix race between dm_get_from_kobject() and __dm_destroy()
3.16.54-rc1 review patch.  If anyone has any objections, please let me know.


From: Hou Tao <>

commit b9a41d21dceadf8104812626ef85dc56ee8a60ed upstream.

The following BUG_ON was hit when testing repeat creation and removal of
DM devices:

kernel BUG at drivers/md/dm.c:2919!
CPU: 7 PID: 750 Comm: systemd-udevd Not tainted 4.1.44
Call Trace:
[<ffffffff81649e8b>] dm_get_from_kobject+0x34/0x3a
[<ffffffff81650ef1>] dm_attr_show+0x2b/0x5e
[<ffffffff817b46d1>] ? mutex_lock+0x26/0x44
[<ffffffff811df7f5>] sysfs_kf_seq_show+0x83/0xcf
[<ffffffff811de257>] kernfs_seq_show+0x23/0x25
[<ffffffff81199118>] seq_read+0x16f/0x325
[<ffffffff811de994>] kernfs_fop_read+0x3a/0x13f
[<ffffffff8117b625>] __vfs_read+0x26/0x9d
[<ffffffff8130eb59>] ? security_file_permission+0x3c/0x44
[<ffffffff8117bdb8>] ? rw_verify_area+0x83/0xd9
[<ffffffff8117be9d>] vfs_read+0x8f/0xcf
[<ffffffff81193e34>] ? __fdget_pos+0x12/0x41
[<ffffffff8117c686>] SyS_read+0x4b/0x76
[<ffffffff817b606e>] system_call_fastpath+0x12/0x71

The bug can be easily triggered, if an extra delay (e.g. 10ms) is added
between the test of DMF_FREEING & DMF_DELETING and dm_get() in

To fix it, we need to ensure the test of DMF_FREEING & DMF_DELETING and
dm_get() are done in an atomic way, so _minor_lock is used.

The other callers of dm_get() have also been checked to be OK: some
callers invoke dm_get() under _minor_lock, some callers invoke it under
_hash_lock, and dm_start_request() invoke it after increasing

Signed-off-by: Hou Tao <>
Signed-off-by: Mike Snitzer <>
Signed-off-by: Ben Hutchings <>
drivers/md/dm.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2912,11 +2912,15 @@ struct mapped_device *dm_get_from_kobjec

md = container_of(kobj, struct mapped_device, kobj_holder.kobj);

- if (test_bit(DMF_FREEING, &md->flags) ||
- dm_deleting_md(md))
- return NULL;
+ spin_lock(&_minor_lock);
+ if (test_bit(DMF_FREEING, &md->flags) || dm_deleting_md(md)) {
+ md = NULL;
+ goto out;
+ }
+ spin_unlock(&_minor_lock);
return md;

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