ocfs2/dlm: Remove BUG_ON from migration in the rare case of a down node

For migration, we are waiting for DLM_LOCK_RES_MIGRATING flag to be set
before sending DLM_MIG_LOCKRES_MSG message to the target. We are using
dlm_migration_can_proceed() for that purpose.  However, if the node is
down, dlm_migration_can_proceed() will also return "go ahead".  In this
rare case, the DLM_LOCK_RES_MIGRATING flag might not be set yet. Remove
the BUG_ON() that trips over this condition.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

fs/ocfs2/dlm/dlmmaster.c

@@ -2808,14 +2808,8 @@ again:
 		mlog(0, "trying again...\n");
 		goto again;
 	}
-	/* now that we are sure the MIGRATING state is there, drop
-	 * the unneded state which blocked threads trying to DIRTY */
-	spin_lock(&res->spinlock);
-	BUG_ON(!(res->state & DLM_LOCK_RES_BLOCK_DIRTY));
-	BUG_ON(!(res->state & DLM_LOCK_RES_MIGRATING));
-	res->state &= ~DLM_LOCK_RES_BLOCK_DIRTY;
-	spin_unlock(&res->spinlock);
 
+	ret = 0;
 	/* did the target go down or die? */
 	spin_lock(&dlm->spinlock);
 	if (!test_bit(target, dlm->domain_map)) {
@@ -2825,10 +2819,22 @@ again:
 	}
 	spin_unlock(&dlm->spinlock);
 
+	/*
+	 * if target is down, we need to clear DLM_LOCK_RES_BLOCK_DIRTY for
+	 * another try; otherwise, we are sure the MIGRATING state is there,
+	 * drop the unneded state which blocked threads trying to DIRTY
+	 */
+	spin_lock(&res->spinlock);
+	BUG_ON(!(res->state & DLM_LOCK_RES_BLOCK_DIRTY));
+	res->state &= ~DLM_LOCK_RES_BLOCK_DIRTY;
+	if (!ret)
+		BUG_ON(!(res->state & DLM_LOCK_RES_MIGRATING));
+	spin_unlock(&res->spinlock);
+
 	/*
 	 * at this point:
 	 *
-	 *   o the DLM_LOCK_RES_MIGRATING flag is set
+	 *   o the DLM_LOCK_RES_MIGRATING flag is set if target not down
 	 *   o there are no pending asts on this lockres
 	 *   o all processes trying to reserve an ast on this
 	 *     lockres must wait for the MIGRATING flag to clear