gfs2: eliminate GLF_QUEUED flag in favor of list_empty(gl_holders)

Before this patch, glock.c maintained a flag, GLF_QUEUED, which indicated
when a glock had a holder queued. It was only checked for inode glocks,
although set and cleared by all glocks, and it was only used to determine
whether the glock should be held for the minimum hold time before releasing.

The problem is that the flag is not accurate at all. If a process holds
the glock, the flag is set. When they dequeue the glock, it only cleared
the flag in cases when the state actually changed. So if the state doesn't
change, the flag may still be set, even when nothing is queued.

This happens to iopen glocks often: the get held in SH, then the file is
closed, but the glock remains in SH mode.

We don't need a special flag to indicate this: we can simply tell whether
the glock has any items queued to the holders queue. It's a waste of cpu
time to maintain it.

This patch eliminates the flag in favor of simply checking list_empty
on the glock holders.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/glock.c

@@ -458,9 +458,6 @@ static void state_change(struct gfs2_glock *gl, unsigned int new_state)
 		else
 			gl->gl_lockref.count--;
 	}
-	if (held1 && held2 && list_empty(&gl->gl_holders))
-		clear_bit(GLF_QUEUED, &gl->gl_flags);
-
 	if (new_state != gl->gl_target)
 		/* shorten our minimum hold time */
 		gl->gl_hold_time = max(gl->gl_hold_time - GL_GLOCK_HOLD_DECR,
@@ -1350,7 +1347,6 @@ __acquires(&gl->gl_lockref.lock)
 		if (unlikely((gh->gh_flags & LM_FLAG_PRIORITY) && !insert_pt))
 			insert_pt = &gh2->gh_list;
 	}
-	set_bit(GLF_QUEUED, &gl->gl_flags);
 	trace_gfs2_glock_queue(gh, 1);
 	gfs2_glstats_inc(gl, GFS2_LKS_QCOUNT);
 	gfs2_sbstats_inc(gl, GFS2_LKS_QCOUNT);
@@ -1651,16 +1647,15 @@ void gfs2_glock_cb(struct gfs2_glock *gl, unsigned int state)
 	unsigned long now = jiffies;
 
 	gfs2_glock_hold(gl);
+	spin_lock(&gl->gl_lockref.lock);
 	holdtime = gl->gl_tchange + gl->gl_hold_time;
-	if (test_bit(GLF_QUEUED, &gl->gl_flags) &&
+	if (!list_empty(&gl->gl_holders) &&
 	    gl->gl_name.ln_type == LM_TYPE_INODE) {
 		if (time_before(now, holdtime))
 			delay = holdtime - now;
 		if (test_bit(GLF_REPLY_PENDING, &gl->gl_flags))
 			delay = gl->gl_hold_time;
 	}
-
-	spin_lock(&gl->gl_lockref.lock);
 	handle_callback(gl, state, delay, true);
 	__gfs2_glock_queue_work(gl, delay);
 	spin_unlock(&gl->gl_lockref.lock);
@@ -2105,7 +2100,7 @@ static const char *gflags2str(char *buf, const struct gfs2_glock *gl)
 		*p++ = 'I';
 	if (test_bit(GLF_FROZEN, gflags))
 		*p++ = 'F';
-	if (test_bit(GLF_QUEUED, gflags))
+	if (!list_empty(&gl->gl_holders))
 		*p++ = 'q';
 	if (test_bit(GLF_LRU, gflags))
 		*p++ = 'L';

fs/gfs2/incore.h

@@ -340,7 +340,6 @@ enum {
 	GLF_REPLY_PENDING		= 9,
 	GLF_INITIAL			= 10,
 	GLF_FROZEN			= 11,
-	GLF_QUEUED			= 12,
 	GLF_LRU				= 13,
 	GLF_OBJECT			= 14, /* Used only for tracing */
 	GLF_BLOCKING			= 15,

fs/gfs2/trace_gfs2.h

@@ -56,7 +56,6 @@
 	{(1UL << GLF_REPLY_PENDING),		"r" },		\
 	{(1UL << GLF_INITIAL),			"I" },		\
 	{(1UL << GLF_FROZEN),			"F" },		\
-	{(1UL << GLF_QUEUED),			"q" },		\
 	{(1UL << GLF_LRU),			"L" },		\
 	{(1UL << GLF_OBJECT),			"o" },		\
 	{(1UL << GLF_BLOCKING),			"b" })