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authorAlex Elder <elder@dreamhost.com>2012-01-29 20:57:43 +0100
committerSage Weil <sage@newdream.net>2012-02-02 21:56:59 +0100
commitd23a4b3fd6ef70b80411b39b8c8bc548a219ce70 (patch)
treef42dbc4c8544acce987e35df5b3002f1def54cff /Documentation
parentrbd: fix a memory leak in rbd_get_client() (diff)
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rbd: fix safety of rbd_put_client()
The rbd_client structure uses a kref to arrange for cleaning up and freeing an instance when its last reference is dropped. The cleanup routine is rbd_client_release(), and one of the things it does is delete the rbd_client from rbd_client_list. It acquires node_lock to do so, but the way it is done is still not safe. The problem is that when attempting to reuse an existing rbd_client, the structure found might already be in the process of getting destroyed and cleaned up. Here's the scenario, with "CLIENT" representing an existing rbd_client that's involved in the race: Thread on CPU A | Thread on CPU B --------------- | --------------- rbd_put_client(CLIENT) | rbd_get_client() kref_put() | (acquires node_lock) kref->refcount becomes 0 | __rbd_client_find() returns CLIENT calls rbd_client_release() | kref_get(&CLIENT->kref); | (releases node_lock) (acquires node_lock) | deletes CLIENT from list | ...and starts using CLIENT... (releases node_lock) | and frees CLIENT | <-- but CLIENT gets freed here Fix this by having rbd_put_client() acquire node_lock. The result could still be improved, but at least it avoids this problem. Signed-off-by: Alex Elder <elder@dreamhost.com> Signed-off-by: Sage Weil <sage@newdream.net>
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