GFS2: Fix glock refcount issues

This patch fixes some ref counting issues. Firstly by moving
the point at which we drop the ref count after a dlm lock
operation has completed we ensure that we never call
gfs2_glock_hold() on a lock with a zero ref count.

Secondly, by using atomic_dec_and_lock() in gfs2_glock_put()
we ensure that at no time will a glock with zero ref count
appear on the lru_list. That means that we can remove the
check for this in our shrinker (which was racy).
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/glock.c

@@ -241,15 +241,14 @@ int gfs2_glock_put(struct gfs2_glock *gl)
 	int rv = 0;
 
 	write_lock(gl_lock_addr(gl->gl_hash));
-	if (atomic_dec_and_test(&gl->gl_ref)) {
+	if (atomic_dec_and_lock(&gl->gl_ref, &lru_lock)) {
 		hlist_del(&gl->gl_list);
-		write_unlock(gl_lock_addr(gl->gl_hash));
-		spin_lock(&lru_lock);
 		if (!list_empty(&gl->gl_lru)) {
 			list_del_init(&gl->gl_lru);
 			atomic_dec(&lru_count);
 		}
 		spin_unlock(&lru_lock);
+		write_unlock(gl_lock_addr(gl->gl_hash));
 		GLOCK_BUG_ON(gl, !list_empty(&gl->gl_holders));
 		glock_free(gl);
 		rv = 1;
@@ -513,7 +512,6 @@ retry:
 			GLOCK_BUG_ON(gl, 1);
 		}
 		spin_unlock(&gl->gl_spin);
-		gfs2_glock_put(gl);
 		return;
 	}
 
@@ -524,8 +522,6 @@ retry:
 		if (glops->go_xmote_bh) {
 			spin_unlock(&gl->gl_spin);
 			rv = glops->go_xmote_bh(gl, gh);
-			if (rv == -EAGAIN)
-				return;
 			spin_lock(&gl->gl_spin);
 			if (rv) {
 				do_error(gl, rv);
@@ -540,7 +536,6 @@ out:
 	clear_bit(GLF_LOCK, &gl->gl_flags);
 out_locked:
 	spin_unlock(&gl->gl_spin);
-	gfs2_glock_put(gl);
 }
 
 static unsigned int gfs2_lm_lock(struct gfs2_sbd *sdp, void *lock,
@@ -600,7 +595,6 @@ __acquires(&gl->gl_spin)
 
 	if (!(ret & LM_OUT_ASYNC)) {
 		finish_xmote(gl, ret);
-		gfs2_glock_hold(gl);
 		if (queue_delayed_work(glock_workqueue, &gl->gl_work, 0) == 0)
 			gfs2_glock_put(gl);
 	} else {
@@ -712,9 +706,12 @@ static void glock_work_func(struct work_struct *work)
 {
 	unsigned long delay = 0;
 	struct gfs2_glock *gl = container_of(work, struct gfs2_glock, gl_work.work);
+	int drop_ref = 0;
 
-	if (test_and_clear_bit(GLF_REPLY_PENDING, &gl->gl_flags))
+	if (test_and_clear_bit(GLF_REPLY_PENDING, &gl->gl_flags)) {
 		finish_xmote(gl, gl->gl_reply);
+		drop_ref = 1;
+	}
 	down_read(&gfs2_umount_flush_sem);
 	spin_lock(&gl->gl_spin);
 	if (test_and_clear_bit(GLF_PENDING_DEMOTE, &gl->gl_flags) &&
@@ -732,6 +729,8 @@ static void glock_work_func(struct work_struct *work)
 	if (!delay ||
 	    queue_delayed_work(glock_workqueue, &gl->gl_work, delay) == 0)
 		gfs2_glock_put(gl);
+	if (drop_ref)
+		gfs2_glock_put(gl);
 }
 
 /**
@@ -1366,10 +1365,6 @@ static int gfs2_shrink_glock_memory(int nr, gfp_t gfp_mask)
 		list_del_init(&gl->gl_lru);
 		atomic_dec(&lru_count);
 
-		/* Check if glock is about to be freed */
-		if (atomic_read(&gl->gl_ref) == 0)
-			continue;
-
 		/* Test for being demotable */
 		if (!test_and_set_bit(GLF_LOCK, &gl->gl_flags)) {
 			gfs2_glock_hold(gl);