rbd: acquire header_rwsem just once in rbd_queue_workfn()

Currently header_rwsem is acquired twice: once in rbd_dev_parent_get()
when the image request is being created and then in rbd_queue_workfn()
to capture mapping_size and snapc.  Introduce rbd_img_capture_header()
and move image request allocation so that header_rwsem can be acquired
just once.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -1601,10 +1601,8 @@ static bool rbd_dev_parent_get(struct rbd_device *rbd_dev)
 	if (!rbd_dev->parent_spec)
 		return false;
 
-	down_read(&rbd_dev->header_rwsem);
 	if (rbd_dev->parent_overlap)
 		counter = atomic_inc_return_safe(&rbd_dev->parent_ref);
-	up_read(&rbd_dev->header_rwsem);
 
 	if (counter < 0)
 		rbd_warn(rbd_dev, "parent reference overflow");
@@ -1619,8 +1617,7 @@ static bool rbd_dev_parent_get(struct rbd_device *rbd_dev)
  */
 static struct rbd_img_request *rbd_img_request_create(
 					struct rbd_device *rbd_dev,
-					enum obj_operation_type op_type,
-					struct ceph_snap_context *snapc)
+					enum obj_operation_type op_type)
 {
 	struct rbd_img_request *img_request;
 
@@ -1630,13 +1627,6 @@ static struct rbd_img_request *rbd_img_request_create(
 
 	img_request->rbd_dev = rbd_dev;
 	img_request->op_type = op_type;
-	if (!rbd_img_is_write(img_request))
-		img_request->snap_id = rbd_dev->spec->snap_id;
-	else
-		img_request->snapc = snapc;
-
-	if (rbd_dev_parent_get(rbd_dev))
-		img_request_layered_set(img_request);
 
 	INIT_LIST_HEAD(&img_request->lock_item);
 	INIT_LIST_HEAD(&img_request->object_extents);
@@ -1645,6 +1635,21 @@ static struct rbd_img_request *rbd_img_request_create(
 	return img_request;
 }
 
+static void rbd_img_capture_header(struct rbd_img_request *img_req)
+{
+	struct rbd_device *rbd_dev = img_req->rbd_dev;
+
+	lockdep_assert_held(&rbd_dev->header_rwsem);
+
+	if (rbd_img_is_write(img_req))
+		img_req->snapc = ceph_get_snap_context(rbd_dev->header.snapc);
+	else
+		img_req->snap_id = rbd_dev->spec->snap_id;
+
+	if (rbd_dev_parent_get(rbd_dev))
+		img_request_layered_set(img_req);
+}
+
 static void rbd_img_request_destroy(struct rbd_img_request *img_request)
 {
 	struct rbd_obj_request *obj_request;
@@ -2825,17 +2830,21 @@ static int rbd_obj_read_object(struct rbd_obj_request *obj_req)
 static int rbd_obj_read_from_parent(struct rbd_obj_request *obj_req)
 {
 	struct rbd_img_request *img_req = obj_req->img_request;
+	struct rbd_device *parent = img_req->rbd_dev->parent;
 	struct rbd_img_request *child_img_req;
 	int ret;
 
-	child_img_req = rbd_img_request_create(img_req->rbd_dev->parent,
-					       OBJ_OP_READ, NULL);
+	child_img_req = rbd_img_request_create(parent, OBJ_OP_READ);
 	if (!child_img_req)
 		return -ENOMEM;
 
 	__set_bit(IMG_REQ_CHILD, &child_img_req->flags);
 	child_img_req->obj_request = obj_req;
 
+	down_read(&parent->header_rwsem);
+	rbd_img_capture_header(child_img_req);
+	up_read(&parent->header_rwsem);
+
 	dout("%s child_img_req %p for obj_req %p\n", __func__, child_img_req,
 	     obj_req);
 
@@ -4686,7 +4695,6 @@ static void rbd_queue_workfn(struct work_struct *work)
 	struct request *rq = blk_mq_rq_from_pdu(work);
 	struct rbd_device *rbd_dev = rq->q->queuedata;
 	struct rbd_img_request *img_request;
-	struct ceph_snap_context *snapc = NULL;
 	u64 offset = (u64)blk_rq_pos(rq) << SECTOR_SHIFT;
 	u64 length = blk_rq_bytes(rq);
 	enum obj_operation_type op_type;
@@ -4739,28 +4747,24 @@ static void rbd_queue_workfn(struct work_struct *work)
 
 	blk_mq_start_request(rq);
 
+	img_request = rbd_img_request_create(rbd_dev, op_type);
+	if (!img_request) {
+		result = -ENOMEM;
+		goto err_rq;
+	}
+	img_request->rq = rq;
+
 	down_read(&rbd_dev->header_rwsem);
 	mapping_size = rbd_dev->mapping.size;
-	if (op_type != OBJ_OP_READ) {
-		snapc = rbd_dev->header.snapc;
-		ceph_get_snap_context(snapc);
-	}
+	rbd_img_capture_header(img_request);
 	up_read(&rbd_dev->header_rwsem);
 
 	if (offset + length > mapping_size) {
 		rbd_warn(rbd_dev, "beyond EOD (%llu~%llu > %llu)", offset,
 			 length, mapping_size);
 		result = -EIO;
-		goto err_rq;
-	}
-
-	img_request = rbd_img_request_create(rbd_dev, op_type, snapc);
-	if (!img_request) {
-		result = -ENOMEM;
-		goto err_rq;
+		goto err_img_request;
 	}
-	img_request->rq = rq;
-	snapc = NULL; /* img_request consumes a ref */
 
 	dout("%s rbd_dev %p img_req %p %s %llu~%llu\n", __func__, rbd_dev,
 	     img_request, obj_op_name(op_type), offset, length);
@@ -4782,7 +4786,6 @@ static void rbd_queue_workfn(struct work_struct *work)
 	if (result)
 		rbd_warn(rbd_dev, "%s %llx at %llx result %d",
 			 obj_op_name(op_type), length, offset, result);
-	ceph_put_snap_context(snapc);
 err:
 	blk_mq_end_request(rq, errno_to_blk_status(result));
 }