[Ocfs2-devel] [PATCH 1/1] o2dlm: fix a race between purge and master query

[Srinivas Eeda]

Node A sends master query request to node B which is the master. At this time
lockres happens to be on purgelist. dlm_master_request_handler gets the dlm
spinlock, finds the resource and releases the dlm spin lock. Right at this
dlm_thread on this node could purge the lockres. dlm_master_request_handler
can then acquire lockres spinlock and reply to Node A that node B is the
master even though lockres on node B is purged.

The above scenario will now make node A falsely think node B is the master
which is inconsistent. Further if another node C tries to master the same
resource, every node will respond they are not the master. Node C then masters
the resource and sends assert master to all nodes. This will now make node A
crash with the following message.

dlm_assert_master_handler:1831 ERROR: DIE! Mastery assert from 9, but current
owner is 10!

Signed-off-by: Srinivas Eeda <srinivas.eeda at oracle.com>
---
 fs/ocfs2/dlm/dlmmaster.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/ocfs2/dlm/dlmmaster.c b/fs/ocfs2/dlm/dlmmaster.c
index 215e41a..3689b35 100644
--- a/fs/ocfs2/dlm/dlmmaster.c
+++ b/fs/ocfs2/dlm/dlmmaster.c
@@ -1460,6 +1460,18 @@ way_up_top:
 
 		/* take care of the easy cases up front */
 		spin_lock(&res->spinlock);
+
+		/*
+		 * Right after dlm spinlock was released, dlm_thread could have
+		 * purged the lockres. Check if lockres got unhashed. If so
+		 * start over.
+		 */
+		if (hlist_unhashed(&res->hash_node)) {
+			spin_unlock(&res->spinlock);
+			dlm_lockres_put(res);
+			goto way_up_top;
+		}
+
 		if (res->state & (DLM_LOCK_RES_RECOVERING|
 				  DLM_LOCK_RES_MIGRATING)) {
 			spin_unlock(&res->spinlock);
-- 
1.9.1