gfs2: Fix possible data races in gfs2_show_options()

Some fields such as gt_logd_secs of the struct gfs2_tune are accessed
without holding the lock gt_spin in gfs2_show_options():

  val = sdp->sd_tune.gt_logd_secs;
  if (val != 30)
    seq_printf(s, ",commit=%d", val);

And thus can cause data races when gfs2_show_options() and other functions
such as gfs2_reconfigure() are concurrently executed:

  spin_lock(&gt->gt_spin);
  gt->gt_logd_secs = newargs->ar_commit;

To fix these possible data races, the lock sdp->sd_tune.gt_spin is
acquired before accessing the fields of gfs2_tune and released after these
accesses.

Further changes by Andreas:

- Don't hold the spin lock over the seq_printf operations.

Reported-by: BassCheck <bass@buaa.edu.cn>
Signed-off-by: Tuo Li <islituo@gmail.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Author: lituo1996

fs/gfs2/super.c

@@ -1004,7 +1004,14 @@ static int gfs2_show_options(struct seq_file *s, struct dentry *root)
 {
 	struct gfs2_sbd *sdp = root->d_sb->s_fs_info;
 	struct gfs2_args *args = &sdp->sd_args;
-	int val;
+	unsigned int logd_secs, statfs_slow, statfs_quantum, quota_quantum;
+
+	spin_lock(&sdp->sd_tune.gt_spin);
+	logd_secs = sdp->sd_tune.gt_logd_secs;
+	quota_quantum = sdp->sd_tune.gt_quota_quantum;
+	statfs_quantum = sdp->sd_tune.gt_statfs_quantum;
+	statfs_slow = sdp->sd_tune.gt_statfs_slow;
+	spin_unlock(&sdp->sd_tune.gt_spin);
 
 	if (is_ancestor(root, sdp->sd_master_dir))
 		seq_puts(s, ",meta");
@@ -1059,17 +1066,14 @@ static int gfs2_show_options(struct seq_file *s, struct dentry *root)
 	}
 	if (args->ar_discard)
 		seq_puts(s, ",discard");
-	val = sdp->sd_tune.gt_logd_secs;
-	if (val != 30)
-		seq_printf(s, ",commit=%d", val);
-	val = sdp->sd_tune.gt_statfs_quantum;
-	if (val != 30)
-		seq_printf(s, ",statfs_quantum=%d", val);
-	else if (sdp->sd_tune.gt_statfs_slow)
+	if (logd_secs != 30)
+		seq_printf(s, ",commit=%d", logd_secs);
+	if (statfs_quantum != 30)
+		seq_printf(s, ",statfs_quantum=%d", statfs_quantum);
+	else if (statfs_slow)
 		seq_puts(s, ",statfs_quantum=0");
-	val = sdp->sd_tune.gt_quota_quantum;
-	if (val != 60)
-		seq_printf(s, ",quota_quantum=%d", val);
+	if (quota_quantum != 60)
+		seq_printf(s, ",quota_quantum=%d", quota_quantum);
 	if (args->ar_statfs_percent)
 		seq_printf(s, ",statfs_percent=%d", args->ar_statfs_percent);
 	if (args->ar_errors != GFS2_ERRORS_DEFAULT) {