drbd: reduce number of spinlock drop/re-aquire cycles

Instead of dropping and re-aquiring the spinlock around the submit,
just remember that we want to submit, and do that only once we have
dropped the spinlock for good.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_req.c

@@ -1086,11 +1086,13 @@ drbd_request_prepare(struct drbd_device *device, struct bio *bio, unsigned long
 
 static void drbd_send_and_submit(struct drbd_device *device, struct drbd_request *req)
 {
+	struct drbd_resource *resource = device->resource;
 	const int rw = bio_rw(req->master_bio);
 	struct bio_and_error m = { NULL, };
 	bool no_remote = false;
+	bool submit_private_bio = false;
 
-	spin_lock_irq(&device->resource->req_lock);
+	spin_lock_irq(&resource->req_lock);
 	if (rw == WRITE) {
 		/* This may temporarily give up the req_lock,
 		 * but will re-aquire it before it returns here.
@@ -1152,9 +1154,7 @@ static void drbd_send_and_submit(struct drbd_device *device, struct drbd_request
 		/* needs to be marked within the same spinlock */
 		_req_mod(req, TO_BE_SUBMITTED);
 		/* but we need to give up the spinlock to submit */
-		spin_unlock_irq(&device->resource->req_lock);
-		drbd_submit_req_private_bio(req);
-		spin_lock_irq(&device->resource->req_lock);
+		submit_private_bio = true;
 	} else if (no_remote) {
 nodata:
 		if (__ratelimit(&drbd_ratelimit_state))
@@ -1167,8 +1167,16 @@ static void drbd_send_and_submit(struct drbd_device *device, struct drbd_request
 out:
 	if (drbd_req_put_completion_ref(req, &m, 1))
 		kref_put(&req->kref, drbd_req_destroy);
-	spin_unlock_irq(&device->resource->req_lock);
-
+	spin_unlock_irq(&resource->req_lock);
+
+	/* Even though above is a kref_put(), this is safe.
+	 * As long as we still need to submit our private bio,
+	 * we hold a completion ref, and the request cannot disappear.
+	 * If however this request did not even have a private bio to submit
+	 * (e.g. remote read), req may already be invalid now.
+	 * That's why we cannot check on req->private_bio. */
+	if (submit_private_bio)
+		drbd_submit_req_private_bio(req);
 	if (m.bio)
 		complete_master_bio(device, &m);
 }