diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
index c7653bb..295c47f 100644
--- a/fs/ocfs2/dlmglue.c
+++ b/fs/ocfs2/dlmglue.c
@@ -311,12 +311,13 @@
 				  struct buffer_head **bh);
 static void ocfs2_drop_osb_locks(struct ocfs2_super *osb);
 static inline int ocfs2_highest_compat_lock_level(int level);
-static void ocfs2_prepare_downconvert(struct ocfs2_lock_res *lockres,
-				      int new_level);
+static unsigned int ocfs2_prepare_downconvert(struct ocfs2_lock_res *lockres,
+					      int new_level);
 static int ocfs2_downconvert_lock(struct ocfs2_super *osb,
 				  struct ocfs2_lock_res *lockres,
 				  int new_level,
-				  int lvb);
+				  int lvb,
+				  unsigned int generation);
 static int ocfs2_prepare_cancel_convert(struct ocfs2_super *osb,
 				        struct ocfs2_lock_res *lockres);
 static int ocfs2_cancel_convert(struct ocfs2_super *osb,
@@ -736,6 +737,113 @@
 	return needs_downconvert;
 }
 
+/*
+ * OCFS2_LOCK_PENDING and l_pending_gen.
+ *
+ * Why does OCFS2_LOCK_PENDING exist?  To close a race between setting
+ * OCFS2_LOCK_BUSY and calling ocfs2_dlm_lock().  See ocfs2_unblock_lock()
+ * for more details on the race.
+ *
+ * OCFS2_LOCK_PENDING closes the race quite nicely.  However, it introduces
+ * a race on itself.  In o2dlm, we can get the ast before ocfs2_dlm_lock()
+ * returns.  The ast clears OCFS2_LOCK_BUSY, and must therefore clear
+ * OCFS2_LOCK_PENDING at the same time.  When ocfs2_dlm_lock() returns,
+ * the caller is going to try to clear PENDING again.  If nothing else is
+ * happening, __lockres_clear_pending() sees PENDING is unset and does
+ * nothing.
+ *
+ * But what if another path (eg downconvert thread) has just started a
+ * new locking action?  The other path has re-set PENDING.  Our path
+ * cannot clear PENDING, because that will re-open the original race
+ * window.
+ *
+ * [Example]
+ *
+ * ocfs2_meta_lock()
+ *  ocfs2_cluster_lock()
+ *   set BUSY
+ *   set PENDING
+ *   drop l_lock
+ *   ocfs2_dlm_lock()
+ *    ocfs2_locking_ast()		ocfs2_downconvert_thread()
+ *     clear PENDING			 ocfs2_unblock_lock()
+ *					  take_l_lock
+ *					  !BUSY
+ *					  ocfs2_prepare_downconvert()
+ *					   set BUSY
+ *					   set PENDING
+ *					  drop l_lock
+ *   take l_lock
+ *   clear PENDING
+ *   drop l_lock
+ *			<window>
+ *					  ocfs2_dlm_lock()
+ *
+ * So as you can see, we now have a window where l_lock is not held,
+ * PENDING is not set, and ocfs2_dlm_lock() has not been called.
+ *
+ * The core problem is that ocfs2_cluster_lock() has cleared the PENDING
+ * set by ocfs2_prepare_downconvert().  That wasn't nice.
+ *
+ * To solve this we introduce l_pending_gen.  A call to
+ * lockres_clear_pending() will only do so when it is passed a generation
+ * number that matches the lockres.  lockres_set_pending() will return the
+ * current generation number.  When ocfs2_cluster_lock() goes to clear
+ * PENDING, it passes the generation it got from set_pending().  In our
+ * example above, the generation numbers will *not* match.  Thus,
+ * ocfs2_cluster_lock() will not clear the PENDING set by
+ * ocfs2_prepare_downconvert().
+ */
+
+/* Unlocked version for ocfs2_locking_ast() */
+static void __lockres_clear_pending(struct ocfs2_lock_res *lockres,
+				    unsigned int generation,
+				    struct ocfs2_super *osb)
+{
+	assert_spin_locked(&lockres->l_lock);
+
+	/*
+	 * The ast and locking functions can race us here.  The winner
+	 * will clear pending, the loser will not.
+	 */
+	if (!(lockres->l_flags & OCFS2_LOCK_PENDING) ||
+	    (lockres->l_pending_gen != generation))
+		return;
+
+	lockres_clear_flags(lockres, OCFS2_LOCK_PENDING);
+	lockres->l_pending_gen++;
+
+	/*
+	 * The downconvert thread may have skipped us because we
+	 * were PENDING.  Wake it up.
+	 */
+	if (lockres->l_flags & OCFS2_LOCK_BLOCKED)
+		ocfs2_wake_downconvert_thread(osb);
+}
+
+/* Locked version for callers of ocfs2_dlm_lock() */
+static void lockres_clear_pending(struct ocfs2_lock_res *lockres,
+				  unsigned int generation,
+				  struct ocfs2_super *osb)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&lockres->l_lock, flags);
+	__lockres_clear_pending(lockres, generation, osb);
+	spin_unlock_irqrestore(&lockres->l_lock, flags);
+}
+
+static unsigned int lockres_set_pending(struct ocfs2_lock_res *lockres)
+{
+	assert_spin_locked(&lockres->l_lock);
+	BUG_ON(!(lockres->l_flags & OCFS2_LOCK_BUSY));
+
+	lockres_or_flags(lockres, OCFS2_LOCK_PENDING);
+
+	return lockres->l_pending_gen;
+}
+
+
 static void ocfs2_blocking_ast(void *opaque, int level)
 {
 	struct ocfs2_lock_res *lockres = opaque;
@@ -770,6 +878,7 @@
 static void ocfs2_locking_ast(void *opaque)
 {
 	struct ocfs2_lock_res *lockres = opaque;
+	struct ocfs2_super *osb = ocfs2_get_lockres_osb(lockres);
 	unsigned long flags;
 
 	spin_lock_irqsave(&lockres->l_lock, flags);
@@ -805,6 +914,18 @@
 	 * can catch it. */
 	lockres->l_action = OCFS2_AST_INVALID;
 
+	/* Did we try to cancel this lock?  Clear that state */
+	if (lockres->l_unlock_action == OCFS2_UNLOCK_CANCEL_CONVERT)
+		lockres->l_unlock_action = OCFS2_UNLOCK_INVALID;
+
+	/*
+	 * We may have beaten the locking functions here.  We certainly
+	 * know that dlm_lock() has been called :-)
+	 * Because we can't have two lock calls in flight at once, we
+	 * can use lockres->l_pending_gen.
+	 */
+	__lockres_clear_pending(lockres, lockres->l_pending_gen,  osb);
+
 	wake_up(&lockres->l_event);
 	spin_unlock_irqrestore(&lockres->l_lock, flags);
 }
@@ -838,6 +959,7 @@
 {
 	int ret = 0;
 	unsigned long flags;
+	unsigned int gen;
 
 	mlog_entry_void();
 
@@ -854,6 +976,7 @@
 	lockres->l_action = OCFS2_AST_ATTACH;
 	lockres->l_requested = level;
 	lockres_or_flags(lockres, OCFS2_LOCK_BUSY);
+	gen = lockres_set_pending(lockres);
 	spin_unlock_irqrestore(&lockres->l_lock, flags);
 
 	ret = ocfs2_dlm_lock(osb->cconn,
@@ -863,6 +986,7 @@
 			     lockres->l_name,
 			     OCFS2_LOCK_ID_MAX_LEN - 1,
 			     lockres);
+	lockres_clear_pending(lockres, gen, osb);
 	if (ret) {
 		ocfs2_log_dlm_error("ocfs2_dlm_lock", ret, lockres);
 		ocfs2_recover_from_dlm_error(lockres, 1);
@@ -988,6 +1112,7 @@
 	int wait, catch_signals = !(osb->s_mount_opt & OCFS2_MOUNT_NOINTR);
 	int ret = 0; /* gcc doesn't realize wait = 1 guarantees ret is set */
 	unsigned long flags;
+	unsigned int gen;
 
 	mlog_entry_void();
 
@@ -1046,6 +1171,7 @@
 
 		lockres->l_requested = level;
 		lockres_or_flags(lockres, OCFS2_LOCK_BUSY);
+		gen = lockres_set_pending(lockres);
 		spin_unlock_irqrestore(&lockres->l_lock, flags);
 
 		BUG_ON(level == DLM_LOCK_IV);
@@ -1062,6 +1188,7 @@
 				     lockres->l_name,
 				     OCFS2_LOCK_ID_MAX_LEN - 1,
 				     lockres);
+		lockres_clear_pending(lockres, gen, osb);
 		if (ret) {
 			if (!(lkm_flags & DLM_LKF_NOQUEUE) ||
 			    (ret != -EAGAIN)) {
@@ -1506,6 +1633,7 @@
 void ocfs2_file_unlock(struct file *file)
 {
 	int ret;
+	unsigned int gen;
 	unsigned long flags;
 	struct ocfs2_file_private *fp = file->private_data;
 	struct ocfs2_lock_res *lockres = &fp->fp_flock;
@@ -1531,11 +1659,11 @@
 	lockres_or_flags(lockres, OCFS2_LOCK_BLOCKED);
 	lockres->l_blocking = DLM_LOCK_EX;
 
-	ocfs2_prepare_downconvert(lockres, LKM_NLMODE);
+	gen = ocfs2_prepare_downconvert(lockres, LKM_NLMODE);
 	lockres_add_mask_waiter(lockres, &mw, OCFS2_LOCK_BUSY, 0);
 	spin_unlock_irqrestore(&lockres->l_lock, flags);
 
-	ret = ocfs2_downconvert_lock(osb, lockres, LKM_NLMODE, 0);
+	ret = ocfs2_downconvert_lock(osb, lockres, LKM_NLMODE, 0, gen);
 	if (ret) {
 		mlog_errno(ret);
 		return;
@@ -2555,23 +2683,7 @@
 	     lockres->l_unlock_action);
 
 	spin_lock_irqsave(&lockres->l_lock, flags);
-	/* We tried to cancel a convert request, but it was already
-	 * granted. All we want to do here is clear our unlock
-	 * state. The wake_up call done at the bottom is redundant
-	 * (ocfs2_prepare_cancel_convert doesn't sleep on this) but doesn't
-	 * hurt anything anyway */
-	if (error == -DLM_ECANCEL &&
-	    lockres->l_unlock_action == OCFS2_UNLOCK_CANCEL_CONVERT) {
-		mlog(0, "Got cancelgrant for %s\n", lockres->l_name);
-
-		/* We don't clear the busy flag in this case as it
-		 * should have been cleared by the ast which the dlm
-		 * has called. */
-		goto complete_unlock;
-	}
-
-	/* DLM_EUNLOCK is the success code for unlock */
-	if (error != -DLM_EUNLOCK) {
+	if (error) {
 		mlog(ML_ERROR, "Dlm passes error %d for lock %s, "
 		     "unlock_action %d\n", error, lockres->l_name,
 		     lockres->l_unlock_action);
@@ -2592,7 +2704,6 @@
 	}
 
 	lockres_clear_flags(lockres, OCFS2_LOCK_BUSY);
-complete_unlock:
 	lockres->l_unlock_action = OCFS2_UNLOCK_INVALID;
 	spin_unlock_irqrestore(&lockres->l_lock, flags);
 
@@ -2768,8 +2879,8 @@
 	return status;
 }
 
-static void ocfs2_prepare_downconvert(struct ocfs2_lock_res *lockres,
-				      int new_level)
+static unsigned int ocfs2_prepare_downconvert(struct ocfs2_lock_res *lockres,
+					      int new_level)
 {
 	assert_spin_locked(&lockres->l_lock);
 
@@ -2787,12 +2898,14 @@
 	lockres->l_action = OCFS2_AST_DOWNCONVERT;
 	lockres->l_requested = new_level;
 	lockres_or_flags(lockres, OCFS2_LOCK_BUSY);
+	return lockres_set_pending(lockres);
 }
 
 static int ocfs2_downconvert_lock(struct ocfs2_super *osb,
 				  struct ocfs2_lock_res *lockres,
 				  int new_level,
-				  int lvb)
+				  int lvb,
+				  unsigned int generation)
 {
 	int ret;
 	u32 dlm_flags = DLM_LKF_CONVERT;
@@ -2809,6 +2922,7 @@
 			     lockres->l_name,
 			     OCFS2_LOCK_ID_MAX_LEN - 1,
 			     lockres);
+	lockres_clear_pending(lockres, generation, osb);
 	if (ret) {
 		ocfs2_log_dlm_error("ocfs2_dlm_lock", ret, lockres);
 		ocfs2_recover_from_dlm_error(lockres, 1);
@@ -2883,6 +2997,7 @@
 	int new_level;
 	int ret = 0;
 	int set_lvb = 0;
+	unsigned int gen;
 
 	mlog_entry_void();
 
@@ -2892,6 +3007,32 @@
 
 recheck:
 	if (lockres->l_flags & OCFS2_LOCK_BUSY) {
+		/* XXX
+		 * This is a *big* race.  The OCFS2_LOCK_PENDING flag
+		 * exists entirely for one reason - another thread has set
+		 * OCFS2_LOCK_BUSY, but has *NOT* yet called dlm_lock().
+		 *
+		 * If we do ocfs2_cancel_convert() before the other thread
+		 * calls dlm_lock(), our cancel will do nothing.  We will
+		 * get no ast, and we will have no way of knowing the
+		 * cancel failed.  Meanwhile, the other thread will call
+		 * into dlm_lock() and wait...forever.
+		 *
+		 * Why forever?  Because another node has asked for the
+		 * lock first; that's why we're here in unblock_lock().
+		 *
+		 * The solution is OCFS2_LOCK_PENDING.  When PENDING is
+		 * set, we just requeue the unblock.  Only when the other
+		 * thread has called dlm_lock() and cleared PENDING will
+		 * we then cancel their request.
+		 *
+		 * All callers of dlm_lock() must set OCFS2_DLM_PENDING
+		 * at the same time they set OCFS2_DLM_BUSY.  They must
+		 * clear OCFS2_DLM_PENDING after dlm_lock() returns.
+		 */
+		if (lockres->l_flags & OCFS2_LOCK_PENDING)
+			goto leave_requeue;
+
 		ctl->requeue = 1;
 		ret = ocfs2_prepare_cancel_convert(osb, lockres);
 		spin_unlock_irqrestore(&lockres->l_lock, flags);
@@ -2971,9 +3112,11 @@
 			lockres->l_ops->set_lvb(lockres);
 	}
 
-	ocfs2_prepare_downconvert(lockres, new_level);
+	gen = ocfs2_prepare_downconvert(lockres, new_level);
 	spin_unlock_irqrestore(&lockres->l_lock, flags);
-	ret = ocfs2_downconvert_lock(osb, lockres, new_level, set_lvb);
+	ret = ocfs2_downconvert_lock(osb, lockres, new_level, set_lvb,
+				     gen);
+
 leave:
 	mlog_exit(ret);
 	return ret;