[GFS2] Handle multiple glock demote requests

Fix a race condition where multiple glock demote requests are sent to
a node back-to-back. This patch does a check inside handle_callback()
to see whether a demote request is in progress. If true, it sets a flag
to make sure run_queue() will loop again to handle the new request,
instead of erronously setting gl_demote_state to a different state.
Signed-off-by: S. Wendy Cheng <wcheng@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/glock.c

@@ -567,7 +567,10 @@ static int rq_demote(struct gfs2_glock *gl)
 		gfs2_demote_wake(gl);
 		return 0;
 	}
+
 	set_bit(GLF_LOCK, &gl->gl_flags);
+	set_bit(GLF_DEMOTE_IN_PROGRESS, &gl->gl_flags);
+
 	if (gl->gl_demote_state == LM_ST_UNLOCKED ||
 	    gl->gl_state != LM_ST_EXCLUSIVE) {
 		spin_unlock(&gl->gl_spin);
@@ -576,7 +579,9 @@ static int rq_demote(struct gfs2_glock *gl)
 		spin_unlock(&gl->gl_spin);
 		gfs2_glock_xmote_th(gl, NULL);
 	}
+
 	spin_lock(&gl->gl_spin);
+	clear_bit(GLF_DEMOTE_IN_PROGRESS, &gl->gl_flags);
 
 	return 0;
 }
@@ -606,6 +611,11 @@ static void run_queue(struct gfs2_glock *gl)
 
 		} else if (test_bit(GLF_DEMOTE, &gl->gl_flags)) {
 			blocked = rq_demote(gl);
+			if (gl->gl_waiters2 && !blocked) {
+				set_bit(GLF_DEMOTE, &gl->gl_flags);
+				gl->gl_demote_state = LM_ST_UNLOCKED;
+			}
+			gl->gl_waiters2 = 0;
 		} else if (!list_empty(&gl->gl_waiters3)) {
 			gh = list_entry(gl->gl_waiters3.next,
 					struct gfs2_holder, gh_list);
@@ -722,7 +732,10 @@ static void handle_callback(struct gfs2_glock *gl, unsigned int state,
 		}
 	} else if (gl->gl_demote_state != LM_ST_UNLOCKED &&
 			gl->gl_demote_state != state) {
-		gl->gl_demote_state = LM_ST_UNLOCKED;
+		if (test_bit(GLF_DEMOTE_IN_PROGRESS,  &gl->gl_flags)) 
+			gl->gl_waiters2 = 1;
+		else 
+			gl->gl_demote_state = LM_ST_UNLOCKED;
 	}
 	spin_unlock(&gl->gl_spin);
 }

fs/gfs2/incore.h

@@ -171,6 +171,7 @@ enum {
 	GLF_DEMOTE		= 3,
 	GLF_PENDING_DEMOTE	= 4,
 	GLF_DIRTY		= 5,
+	GLF_DEMOTE_IN_PROGRESS	= 6,
 };
 
 struct gfs2_glock {
@@ -190,6 +191,7 @@ struct gfs2_glock {
 	struct list_head gl_holders;
 	struct list_head gl_waiters1;	/* HIF_MUTEX */
 	struct list_head gl_waiters3;	/* HIF_PROMOTE */
+	int gl_waiters2;		/* GIF_DEMOTE */
 
 	const struct gfs2_glock_operations *gl_ops;