[Virtio-fs] [PATCH] virtiofs: reduce lock contention on fpq->lock

[Liu Bo]

Right now within virtio_fs_requests_done_work(), virtqueue is protected by
both fsvq->lock and fpq->lock for processing both virtqueue and
fpq->processing_list, however, it is fine for them to be protected
separately.

Also since %req has been removed from fpq->processing_list, no one except
request_wait_answer() is looking at this %req and request_wait_answer()
waits only on FINISH flag, it's OK to remove fpq->lock after %req is
dropped from the list.

Signed-off-by: Liu Bo <bo.liu at linux.alibaba.com>
---
 fs/fuse/virtio_fs.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 9b3f2e9..d88bb6c 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -347,21 +347,22 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
 
 	/* Collect completed requests off the virtqueue */
 	spin_lock(&fsvq->lock);
-	/* grab fpq->lock as req is on fpq->processing list */
-	spin_lock(&fpq->lock);
 	do {
 		unsigned len;
 
 		virtqueue_disable_cb(vq);
 
 		while ((req = virtqueue_get_buf(vq, &len)) != NULL) {
+			/* grab fpq->lock as req is on fpq->processing list */
+			spin_lock(&fpq->lock);
+
 			/* If !connected, req is freed by fuse_abort_conn. */
-			if (fpq->connected) {
+			if (fpq->connected)
 				list_move_tail(&req->list, &reqs);
-			}
+
+			spin_unlock(&fpq->lock);
 		}
 	} while (!virtqueue_enable_cb(vq) && likely(!virtqueue_is_broken(vq)));
-	spin_unlock(&fpq->lock);
 	spin_unlock(&fsvq->lock);
 
 	/* End requests */
@@ -373,10 +374,8 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
 
 		/* TODO zeroing? */
 
-		spin_lock(&fpq->lock);
 		clear_bit(FR_SENT, &req->flags);
 		list_del_init(&req->list);
-		spin_unlock(&fpq->lock);
 
 		fuse_request_end(fc, req);
 	}
-- 
1.8.3.1