gfs2: Fix lru_count accounting

Currently, gfs2_scan_glock_lru() decrements lru_count when a glock is
moved onto the dispose list.  When such a glock is then stolen from the
dispose list while gfs2_dispose_glock_lru() doesn't hold the lru_lock,
lru_count will be decremented again, so the counter will eventually go
negative.

This bug has existed in one form or another since at least commit
97cc1025 ("GFS2: Kill two daemons with one patch").

Fix this by only decrementing lru_count when we actually remove a glock
and schedule for it to be unlocked and dropped.  We also don't need to
remove and then re-add glocks when we can just as well move them back
onto the lru_list when necessary.

In addition, return the number of glocks freed as we should, not the
number of glocks moved onto the dispose list.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/glock.c

@@ -2009,29 +2009,30 @@ static bool can_free_glock(struct gfs2_glock *gl)
  * private)
  */
 
-static void gfs2_dispose_glock_lru(struct list_head *list)
+static unsigned long gfs2_dispose_glock_lru(struct list_head *list)
 __releases(&lru_lock)
 __acquires(&lru_lock)
 {
 	struct gfs2_glock *gl;
+	unsigned long freed = 0;
 
 	list_sort(NULL, list, glock_cmp);
 
 	while(!list_empty(list)) {
 		gl = list_first_entry(list, struct gfs2_glock, gl_lru);
-		list_del_init(&gl->gl_lru);
-		clear_bit(GLF_LRU, &gl->gl_flags);
 		if (!spin_trylock(&gl->gl_lockref.lock)) {
 add_back_to_lru:
-			list_add(&gl->gl_lru, &lru_list);
-			set_bit(GLF_LRU, &gl->gl_flags);
-			atomic_inc(&lru_count);
+			list_move(&gl->gl_lru, &lru_list);
 			continue;
 		}
 		if (!can_free_glock(gl)) {
 			spin_unlock(&gl->gl_lockref.lock);
 			goto add_back_to_lru;
 		}
+		list_del_init(&gl->gl_lru);
+		atomic_dec(&lru_count);
+		clear_bit(GLF_LRU, &gl->gl_flags);
+		freed++;
 		gl->gl_lockref.count++;
 		if (demote_ok(gl))
 			handle_callback(gl, LM_ST_UNLOCKED, 0, false);
@@ -2039,6 +2040,7 @@ __acquires(&lru_lock)
 		spin_unlock(&gl->gl_lockref.lock);
 		cond_resched_lock(&lru_lock);
 	}
+	return freed;
 }
 
 /**
@@ -2050,24 +2052,21 @@ __acquires(&lru_lock)
  * gfs2_dispose_glock_lru() above.
  */
 
-static long gfs2_scan_glock_lru(int nr)
+static unsigned long gfs2_scan_glock_lru(unsigned long nr)
 {
 	struct gfs2_glock *gl, *next;
 	LIST_HEAD(dispose);
-	long freed = 0;
+	unsigned long freed = 0;
 
 	spin_lock(&lru_lock);
 	list_for_each_entry_safe(gl, next, &lru_list, gl_lru) {
-		if (nr-- <= 0)
+		if (!nr--)
 			break;
-		if (can_free_glock(gl)) {
+		if (can_free_glock(gl))
 			list_move(&gl->gl_lru, &dispose);
-			atomic_dec(&lru_count);
-			freed++;
-		}
 	}
 	if (!list_empty(&dispose))
-		gfs2_dispose_glock_lru(&dispose);
+		freed = gfs2_dispose_glock_lru(&dispose);
 	spin_unlock(&lru_lock);
 
 	return freed;