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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@ -407,15 +407,15 @@ done_err:
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/*
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* Destroy ceph client
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*
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* Caller must hold node_lock.
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*/
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static void rbd_client_release(struct kref *kref)
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{
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struct rbd_client *rbdc = container_of(kref, struct rbd_client, kref);
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dout("rbd_release_client %p\n", rbdc);
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spin_lock(&node_lock);
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list_del(&rbdc->node);
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spin_unlock(&node_lock);
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ceph_destroy_client(rbdc->client);
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kfree(rbdc->rbd_opts);
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@ -428,7 +428,9 @@ static void rbd_client_release(struct kref *kref)
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*/
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static void rbd_put_client(struct rbd_device *rbd_dev)
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{
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spin_lock(&node_lock);
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kref_put(&rbd_dev->rbd_client->kref, rbd_client_release);
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spin_unlock(&node_lock);
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rbd_dev->rbd_client = NULL;
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rbd_dev->client = NULL;
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}
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