fuse: add locking to max_background and congestion_threshold changes

Functions sequences like request_end()->flush_bg_queue() require that
max_background and congestion_threshold are constant during their
execution. Otherwise, checks like

	if (fc->num_background == fc->max_background)

made in different time may behave not like expected.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/control.c

@@ -125,7 +125,12 @@ static ssize_t fuse_conn_max_background_write(struct file *file,
 	if (ret > 0) {
 		struct fuse_conn *fc = fuse_ctl_file_conn_get(file);
 		if (fc) {
+			spin_lock(&fc->lock);
 			fc->max_background = val;
+			fc->blocked = fc->num_background >= fc->max_background;
+			if (!fc->blocked)
+				wake_up(&fc->blocked_waitq);
+			spin_unlock(&fc->lock);
 			fuse_conn_put(fc);
 		}
 	}
@@ -155,18 +160,31 @@ static ssize_t fuse_conn_congestion_threshold_write(struct file *file,
 						    size_t count, loff_t *ppos)
 {
 	unsigned uninitialized_var(val);
+	struct fuse_conn *fc;
 	ssize_t ret;
 
 	ret = fuse_conn_limit_write(file, buf, count, ppos, &val,
 				    max_user_congthresh);
-	if (ret > 0) {
-		struct fuse_conn *fc = fuse_ctl_file_conn_get(file);
-		if (fc) {
-			fc->congestion_threshold = val;
-			fuse_conn_put(fc);
+	if (ret <= 0)
+		goto out;
+	fc = fuse_ctl_file_conn_get(file);
+	if (!fc)
+		goto out;
+
+	spin_lock(&fc->lock);
+	fc->congestion_threshold = val;
+	if (fc->sb) {
+		if (fc->num_background < fc->congestion_threshold) {
+			clear_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC);
+			clear_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC);
+		} else {
+			set_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC);
+			set_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC);
 		}
 	}
-
+	spin_unlock(&fc->lock);
+	fuse_conn_put(fc);
+out:
 	return ret;
 }