gfs2: Fix debugfs glocks dump

commit 10201655 upstream.

The switch to rhashtables (commit 88ffbf3e) broke the debugfs glock
dump (/sys/kernel/debug/gfs2/<device>/glocks) for dumps bigger than a
single buffer: the right function for restarting an rhashtable iteration
from the beginning of the hash table is rhashtable_walk_enter;
rhashtable_walk_stop + rhashtable_walk_start will just resume from the
current position.

The upstream commit doesn't directly apply to 4.4.y because 4.4.y
doesn't have rhashtable_walk_enter and the following mainline commits:

  92ecd73a  
    gfs2: Deduplicate gfs2_{glocks,glstats}_open
  cc37a627  
    gfs2: Replace rhashtable_walk_init with rhashtable_walk_enter

Other than rhashtable_walk_enter, rhashtable_walk_init can fail.  To
handle the failure case in gfs2_glock_seq_stop, we check if
rhashtable_walk_init has initialized iter->walker; if it has not, we
must not call rhashtable_walk_stop or rhashtable_walk_exit.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/gfs2/glock.c

@@ -1814,13 +1814,10 @@ static void *gfs2_glock_seq_start(struct seq_file *seq, loff_t *pos)
 {
 	struct gfs2_glock_iter *gi = seq->private;
 	loff_t n = *pos;
-	int ret;
-
-	if (gi->last_pos <= *pos)
-		n = (*pos - gi->last_pos);
 
-	ret = rhashtable_walk_start(&gi->hti);
-	if (ret)
+	if (rhashtable_walk_init(&gl_hash_table, &gi->hti) != 0)
+		return NULL;
+	if (rhashtable_walk_start(&gi->hti) != 0)
 		return NULL;
 
 	do {
@@ -1828,6 +1825,7 @@ static void *gfs2_glock_seq_start(struct seq_file *seq, loff_t *pos)
 	} while (gi->gl && n--);
 
 	gi->last_pos = *pos;
+
 	return gi->gl;
 }
 
@@ -1839,6 +1837,7 @@ static void *gfs2_glock_seq_next(struct seq_file *seq, void *iter_ptr,
 	(*pos)++;
 	gi->last_pos = *pos;
 	gfs2_glock_iter_next(gi);
+
 	return gi->gl;
 }
 
@@ -1847,7 +1846,10 @@ static void gfs2_glock_seq_stop(struct seq_file *seq, void *iter_ptr)
 	struct gfs2_glock_iter *gi = seq->private;
 
 	gi->gl = NULL;
-	rhashtable_walk_stop(&gi->hti);
+	if (gi->hti.walker) {
+		rhashtable_walk_stop(&gi->hti);
+		rhashtable_walk_exit(&gi->hti);
+	}
 }
 
 static int gfs2_glock_seq_show(struct seq_file *seq, void *iter_ptr)
@@ -1910,12 +1912,10 @@ static int gfs2_glocks_open(struct inode *inode, struct file *file)
 		struct gfs2_glock_iter *gi = seq->private;
 
 		gi->sdp = inode->i_private;
-		gi->last_pos = 0;
 		seq->buf = kmalloc(GFS2_SEQ_GOODSIZE, GFP_KERNEL | __GFP_NOWARN);
 		if (seq->buf)
 			seq->size = GFS2_SEQ_GOODSIZE;
 		gi->gl = NULL;
-		ret = rhashtable_walk_init(&gl_hash_table, &gi->hti);
 	}
 	return ret;
 }
@@ -1926,7 +1926,6 @@ static int gfs2_glocks_release(struct inode *inode, struct file *file)
 	struct gfs2_glock_iter *gi = seq->private;
 
 	gi->gl = NULL;
-	rhashtable_walk_exit(&gi->hti);
 	return seq_release_private(inode, file);
 }
 
@@ -1938,12 +1937,10 @@ static int gfs2_glstats_open(struct inode *inode, struct file *file)
 		struct seq_file *seq = file->private_data;
 		struct gfs2_glock_iter *gi = seq->private;
 		gi->sdp = inode->i_private;
-		gi->last_pos = 0;
 		seq->buf = kmalloc(GFS2_SEQ_GOODSIZE, GFP_KERNEL | __GFP_NOWARN);
 		if (seq->buf)
 			seq->size = GFS2_SEQ_GOODSIZE;
 		gi->gl = NULL;
-		ret = rhashtable_walk_init(&gl_hash_table, &gi->hti);
 	}
 	return ret;
 }