gfs2: Check for empty queue in run_queue

[ Upstream commit d838605 ]

In run_queue(), check if the queue of pending requests is empty instead
of blindly assuming that it won't be.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Andreas Gruenbacher

fs/gfs2/glock.c

@@ -843,12 +843,13 @@ static void run_queue(struct gfs2_glock *gl, const int nonblock)
 __releases(&gl->gl_lockref.lock)
 __acquires(&gl->gl_lockref.lock)
 {
-	struct gfs2_holder *gh = NULL;
+	struct gfs2_holder *gh;
 
 	if (test_bit(GLF_LOCK, &gl->gl_flags))
 		return;
 	set_bit(GLF_LOCK, &gl->gl_flags);
 
+	/* While a demote is in progress, the GLF_LOCK flag must be set. */
 	GLOCK_BUG_ON(gl, test_bit(GLF_DEMOTE_IN_PROGRESS, &gl->gl_flags));
 
 	if (test_bit(GLF_DEMOTE, &gl->gl_flags) &&
@@ -860,18 +861,22 @@ __acquires(&gl->gl_lockref.lock)
 		set_bit(GLF_DEMOTE_IN_PROGRESS, &gl->gl_flags);
 		GLOCK_BUG_ON(gl, gl->gl_demote_state == LM_ST_EXCLUSIVE);
 		gl->gl_target = gl->gl_demote_state;
+		do_xmote(gl, NULL, gl->gl_target);
+		return;
 	} else {
 		if (test_bit(GLF_DEMOTE, &gl->gl_flags))
 			gfs2_demote_wake(gl);
 		if (do_promote(gl))
 			goto out_unlock;
 		gh = find_first_waiter(gl);
+		if (!gh)
+			goto out_unlock;
 		gl->gl_target = gh->gh_state;
 		if (!(gh->gh_flags & (LM_FLAG_TRY | LM_FLAG_TRY_1CB)))
 			do_error(gl, 0); /* Fail queued try locks */
+		do_xmote(gl, gh, gl->gl_target);
+		return;
 	}
-	do_xmote(gl, gh, gl->gl_target);
-	return;
 
 out_sched:
 	clear_bit(GLF_LOCK, &gl->gl_flags);