Re: [PATCH 6/6] fuse: Do not take fuse_conn::lock on fuse_request_send_background()

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, Aug 27, 2018 at 06:29:56PM +0300, Kirill Tkhai wrote:
> Currently, we take fc->lock there only to check for fc->connected.
> But this flag is changed only on connection abort, which is very
> rare operation. Good thing looks to make fuse_request_send_background()
> faster, while fuse_abort_conn() slowler.
> 
> So, we make fuse_request_send_background() lockless and mark
> (fc->connected == 1) region as RCU-protected. Abort function
> just uses synchronize_sched() to wait till all pending background
> requests is being queued, and then makes ordinary abort.
> 
> Note, that synchronize_sched() is used instead of synchronize_rcu(),
> since we want to check for fc->connected without rcu_dereference()
> in fuse_request_send_background() (i.e., not to add memory barriers
> to this hot path).

Apart from the inaccuracies in the above (_sched variant is for scheduling and
NMI taking code; _sched variant requires rcu_dereference() as well;
rcu_dereference() does not add barriers; rcu_dereference() is only for pointers,
so we can't use it for an integer), wouldn't it be simpler to just use bg_lock
for checking ->connected, and lock bg_lock (as well as fc->lock) when setting
->connected?

Updated patch below (untested).

Thanks,
Miklos

---
Subject: fuse: do not take fc->lock in fuse_request_send_background()
From: Kirill Tkhai <ktkhai@virtuozzo.com>
Date: Mon, 27 Aug 2018 18:29:56 +0300

Currently, we take fc->lock there only to check for fc->connected.
But this flag is changed only on connection abort, which is very
rare operation.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/fuse/dev.c    |   46 +++++++++++++++++++++++-----------------------
 fs/fuse/file.c   |    4 +++-
 fs/fuse/fuse_i.h |    4 +---
 3 files changed, 27 insertions(+), 27 deletions(-)

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -574,42 +574,38 @@ ssize_t fuse_simple_request(struct fuse_
 	return ret;
 }
 
-/*
- * Called under fc->lock
- *
- * fc->connected must have been checked previously
- */
-void fuse_request_send_background_nocheck(struct fuse_conn *fc,
-					  struct fuse_req *req)
+bool fuse_request_queue_background(struct fuse_conn *fc, struct fuse_req *req)
 {
-	BUG_ON(!test_bit(FR_BACKGROUND, &req->flags));
+	bool queued = false;
+
+	WARN_ON(!test_bit(FR_BACKGROUND, &req->flags));
 	if (!test_bit(FR_WAITING, &req->flags)) {
 		__set_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;
-	if (fc->num_background == fc->congestion_threshold && fc->sb) {
-		set_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC);
-		set_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC);
+	if (likely(fc->connected)) {
+		fc->num_background++;
+		if (fc->num_background == fc->max_background)
+			fc->blocked = 1;
+		if (fc->num_background == fc->congestion_threshold && fc->sb) {
+			set_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC);
+			set_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC);
+		}
+		list_add_tail(&req->list, &fc->bg_queue);
+		flush_bg_queue(fc);
+		queued = true;
 	}
-	list_add_tail(&req->list, &fc->bg_queue);
-	flush_bg_queue(fc);
 	spin_unlock(&fc->bg_lock);
+
+	return queued;
 }
 
 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_nocheck(fc, req);
-		spin_unlock(&fc->lock);
-	} else {
-		spin_unlock(&fc->lock);
+	WARN_ON(!req->end);
+	if (!fuse_request_queue_background(fc, req)) {
 		req->out.h.error = -ENOTCONN;
 		req->end(fc, req);
 		fuse_put_request(fc, req);
@@ -2112,7 +2108,11 @@ void fuse_abort_conn(struct fuse_conn *f
 		struct fuse_req *req, *next;
 		LIST_HEAD(to_end);
 
+		/* Background queuing checks fc->connected under bg_lock */
+		spin_lock(&fc->bg_lock);
 		fc->connected = 0;
+		spin_unlock(&fc->bg_lock);
+
 		fc->aborted = is_abort;
 		fuse_set_initialized(fc);
 		list_for_each_entry(fud, &fc->devices, entry) {
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -863,9 +863,7 @@ ssize_t fuse_simple_request(struct fuse_
  * Send a request in the background
  */
 void fuse_request_send_background(struct fuse_conn *fc, struct fuse_req *req);
-
-void fuse_request_send_background_nocheck(struct fuse_conn *fc,
-					  struct fuse_req *req);
+bool fuse_request_queue_background(struct fuse_conn *fc, struct fuse_req *req);
 
 /* Abort all requests */
 void fuse_abort_conn(struct fuse_conn *fc, bool is_abort);
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1487,6 +1487,7 @@ __acquires(fc->lock)
 	struct fuse_inode *fi = get_fuse_inode(req->inode);
 	struct fuse_write_in *inarg = &req->misc.write.in;
 	__u64 data_size = req->num_pages * PAGE_SIZE;
+	bool queued;
 
 	if (!fc->connected)
 		goto out_free;
@@ -1502,7 +1503,8 @@ __acquires(fc->lock)
 
 	req->in.args[1].size = inarg->size;
 	fi->writectr++;
-	fuse_request_send_background_nocheck(fc, req);
+	queued = fuse_request_queue_background(fc, req);
+	WARN_ON(!queued);
 	return;
 
  out_free: