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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
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:
