[Devel] [PATCH 2/3] fuse: Optimize request_end() by not taking fiq->waitq.lock

[Kirill Tkhai]

ms commit 217316a60101

We take global fiq->waitq.lock every time, when we are in this function,
but interrupted requests are just small subset of all requests. This patch
optimizes request_end() and makes it to take the lock when it's really
needed.

queue_interrupt() needs small change for that. After req is linked to
interrupt list, we do smp_mb() and check for FR_FINISHED again. In case of
FR_FINISHED bit has appeared, we remove req and leave the function:

Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi at redhat.com>
Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 fs/fuse/dev.c |   28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index eb5c62e3170f..18379fa9ca12 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -425,11 +425,19 @@ void request_end(struct fuse_conn *fc, struct fuse_req *req)
 	if (test_and_set_bit(FR_FINISHED, &req->flags))
 		return;
 
+	/*
+	 * test_and_set_bit() implies smp_mb() between bit
+	 * changing and below intr_entry check. Pairs with
+	 * smp_mb() from queue_interrupt().
+	 */
+	if (!list_empty(&req->intr_entry)) {
+		spin_lock(&fiq->waitq.lock);
+		list_del_init(&req->intr_entry);
+		spin_unlock(&fiq->waitq.lock);
+	}
+
 	bg = test_bit(FR_BACKGROUND, &req->flags);
 
-	spin_lock(&fiq->waitq.lock);
-	list_del_init(&req->intr_entry);
-	spin_unlock(&fiq->waitq.lock);
 	WARN_ON(test_bit(FR_PENDING, &req->flags));
 	WARN_ON(test_bit(FR_SENT, &req->flags));
 	if (bg) {
@@ -472,12 +480,18 @@ EXPORT_SYMBOL_GPL(request_end);
 static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
 {
 	spin_lock(&fiq->waitq.lock);
-	if (test_bit(FR_FINISHED, &req->flags)) {
-		spin_unlock(&fiq->waitq.lock);
-		return;
-	}
 	if (list_empty(&req->intr_entry)) {
 		list_add_tail(&req->intr_entry, &fiq->interrupts);
+		/*
+		 * Pairs with smp_mb() implied by test_and_set_bit()
+		 * from request_end().
+		 */
+		smp_mb();
+		if (test_bit(FR_FINISHED, &req->flags)) {
+			list_del_init(&req->intr_entry);
+			spin_unlock(&fiq->waitq.lock);
+			return;
+		}
 		wake_up_locked(&fiq->waitq);
 		kill_fasync(&fiq->fasync, SIGIO, POLL_IN);
 	}