fuse: introduce fc->bg_lock

To reduce contention of fc->lock, this patch introduces bg_lock for
protection of fields related to background queue. These are:
max_background, congestion_threshold, num_background, active_background,
bg_queue and blocked.

This allows next patch to make async reads not requiring fc->lock, so async
reads and writes will have better performance executed in parallel.

Signed-off-by: Kirill Tkhai <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>

fs/fuse/control.c

···
 	if (ret > 0) {
 		struct fuse_conn *fc = fuse_ctl_file_conn_get(file);
 		if (fc) {
-			spin_lock(&fc->lock);
+			spin_lock(&fc->bg_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);
+			spin_unlock(&fc->bg_lock);
 			fuse_conn_put(fc);
 		}
 	}
···
 	if (!fc)
 		goto out;
 
-	spin_lock(&fc->lock);
+	spin_lock(&fc->bg_lock);
 	fc->congestion_threshold = val;
 	if (fc->sb) {
 		if (fc->num_background < fc->congestion_threshold) {
···
 			set_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC);
 		}
 	}
-	spin_unlock(&fc->lock);
+	spin_unlock(&fc->bg_lock);
 	fuse_conn_put(fc);
 out:
 	return ret;

fs/fuse/dev.c

···
 			 * We get here in the unlikely case that a background
 			 * request was allocated but not sent
 			 */
-			spin_lock(&fc->lock);
+			spin_lock(&fc->bg_lock);
 			if (!fc->blocked)
 				wake_up(&fc->blocked_waitq);
-			spin_unlock(&fc->lock);
+			spin_unlock(&fc->bg_lock);
 		}
 
 		if (test_bit(FR_WAITING, &req->flags)) {
···
 	WARN_ON(test_bit(FR_PENDING, &req->flags));
 	WARN_ON(test_bit(FR_SENT, &req->flags));
 	if (test_bit(FR_BACKGROUND, &req->flags)) {
-		spin_lock(&fc->lock);
+		spin_lock(&fc->bg_lock);
 		clear_bit(FR_BACKGROUND, &req->flags);
 		if (fc->num_background == fc->max_background) {
 			fc->blocked = 0;
···
 		fc->num_background--;
 		fc->active_background--;
 		flush_bg_queue(fc);
-		spin_unlock(&fc->lock);
+		spin_unlock(&fc->bg_lock);
 	}
 	wake_up(&req->waitq);
 	if (req->end)
···
  *
  * fc->connected must have been checked previously
  */
-void fuse_request_send_background_locked(struct fuse_conn *fc,
-					 struct fuse_req *req)
+void fuse_request_send_background_nocheck(struct fuse_conn *fc,
+					  struct fuse_req *req)
 {
 	BUG_ON(!test_bit(FR_BACKGROUND, &req->flags));
 	if (!test_bit(FR_WAITING, &req->flags)) {
···
 		atomic_inc(&fc->num_waiting);
 	}
 	__set_bit(FR_ISREPLY, &req->flags);
+	spin_lock(&fc->bg_lock);
 	fc->num_background++;
 	if (fc->num_background == fc->max_background)
 		fc->blocked = 1;
···
 	}
 	list_add_tail(&req->list, &fc->bg_queue);
 	flush_bg_queue(fc);
+	spin_unlock(&fc->bg_lock);
 }
 
 void fuse_request_send_background(struct fuse_conn *fc, struct fuse_req *req)
···
 	BUG_ON(!req->end);
 	spin_lock(&fc->lock);
 	if (fc->connected) {
-		fuse_request_send_background_locked(fc, req);
+		fuse_request_send_background_nocheck(fc, req);
 		spin_unlock(&fc->lock);
 	} else {
 		spin_unlock(&fc->lock);
···
 		LIST_HEAD(to_end);
 
 		fc->connected = 0;
-		fc->blocked = 0;
 		fc->aborted = is_abort;
 		fuse_set_initialized(fc);
 		list_for_each_entry(fud, &fc->devices, entry) {
···
 			list_splice_tail_init(&fpq->processing, &to_end);
 			spin_unlock(&fpq->lock);
 		}
+		spin_lock(&fc->bg_lock);
+		fc->blocked = 0;
 		fc->max_background = UINT_MAX;
 		flush_bg_queue(fc);
+		spin_unlock(&fc->bg_lock);
 
 		spin_lock(&fiq->waitq.lock);
 		fiq->connected = 0;

fs/fuse/file.c

···
 
 	req->in.args[1].size = inarg->size;
 	fi->writectr++;
-	fuse_request_send_background_locked(fc, req);
+	fuse_request_send_background_nocheck(fc, req);
 	return;
 
  out_free:

fs/fuse/fuse_i.h

···
 	/** The list of background requests set aside for later queuing */
 	struct list_head bg_queue;
 
+	/** Protects: max_background, congestion_threshold, num_background,
+	 * active_background, bg_queue, blocked */
+	spinlock_t bg_lock;
+
 	/** Flag indicating that INIT reply has been received. Allocating
 	 * any fuse request will be suspended until the flag is set */
 	int initialized;
···
  */
 void fuse_request_send_background(struct fuse_conn *fc, struct fuse_req *req);
 
-void fuse_request_send_background_locked(struct fuse_conn *fc,
-					 struct fuse_req *req);
+void fuse_request_send_background_nocheck(struct fuse_conn *fc,
+					  struct fuse_req *req);
 
 /* Abort all requests */
 void fuse_abort_conn(struct fuse_conn *fc, bool is_abort);

fs/fuse/inode.c

···
 {
 	memset(fc, 0, sizeof(*fc));
 	spin_lock_init(&fc->lock);
+	spin_lock_init(&fc->bg_lock);
 	init_rwsem(&fc->killsb);
 	refcount_set(&fc->count, 1);
 	atomic_set(&fc->dev_count, 1);
···
 	sanitize_global_limit(&max_user_bgreq);
 	sanitize_global_limit(&max_user_congthresh);
 
+	spin_lock(&fc->bg_lock);
 	if (arg->max_background) {
 		fc->max_background = arg->max_background;
 
···
 		    fc->congestion_threshold > max_user_congthresh)
 			fc->congestion_threshold = max_user_congthresh;
 	}
+	spin_unlock(&fc->bg_lock);
 }
 
 static void process_init_reply(struct fuse_conn *fc, struct fuse_req *req)