rbd: move snap_rwsem to the device, rename to header_rwsem

A new temporary header is allocated each time the header changes, but
only the changed properties are copied over. We don't need a new
semaphore for each header update.

This addresses http://tracker.newdream.net/issues/2174

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Alex Elder <elder@dreamhost.com>

drivers/block/rbd.c

@@ -82,7 +82,6 @@ struct rbd_image_header {
 	__u8 obj_order;
 	__u8 crypt_type;
 	__u8 comp_type;
-	struct rw_semaphore snap_rwsem;
 	struct ceph_snap_context *snapc;
 	size_t snap_names_len;
 	u64 snap_seq;
@@ -173,6 +172,8 @@ struct rbd_device {
 	struct ceph_osd_event   *watch_event;
 	struct ceph_osd_request *watch_request;
 
+	/* protects updating the header */
+	struct rw_semaphore     header_rwsem;
 	char                    snap_name[RBD_MAX_SNAP_NAME_LEN];
 	u32 cur_snap;	/* index+1 of current snapshot within snap context
 			   0 - for the head */
@@ -502,7 +503,6 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 	if (!header->snapc)
 		return -ENOMEM;
 
-	init_rwsem(&header->snap_rwsem);
 	header->snap_names_len = le64_to_cpu(ondisk->snap_names_len);
 	if (snap_count) {
 		header->snap_names = kmalloc(header->snap_names_len,
@@ -597,7 +597,7 @@ static int rbd_header_set_snap(struct rbd_device *dev, u64 *size)
 
 	BUILD_BUG_ON(sizeof (dev->snap_name) < sizeof (RBD_SNAP_HEAD_NAME));
 
-	down_write(&header->snap_rwsem);
+	down_write(&dev->header_rwsem);
 
 	if (!memcmp(dev->snap_name, RBD_SNAP_HEAD_NAME,
 		    sizeof (RBD_SNAP_HEAD_NAME))) {
@@ -620,7 +620,7 @@ static int rbd_header_set_snap(struct rbd_device *dev, u64 *size)
 
 	ret = 0;
 done:
-	up_write(&header->snap_rwsem);
+	up_write(&dev->header_rwsem);
 	return ret;
 }
 
@@ -887,7 +887,6 @@ static int rbd_do_request(struct request *rq,
 	struct timespec mtime = CURRENT_TIME;
 	struct rbd_request *req_data;
 	struct ceph_osd_request_head *reqhead;
-	struct rbd_image_header *header = &dev->header;
 	struct ceph_osd_client *osdc;
 
 	req_data = kzalloc(sizeof(*req_data), GFP_NOIO);
@@ -905,13 +904,13 @@ static int rbd_do_request(struct request *rq,
 
 	dout("rbd_do_request obj=%s ofs=%lld len=%lld\n", obj, len, ofs);
 
-	down_read(&header->snap_rwsem);
+	down_read(&dev->header_rwsem);
 
 	osdc = &dev->rbd_client->client->osdc;
 	req = ceph_osdc_alloc_request(osdc, flags, snapc, ops,
 					false, GFP_NOIO, pages, bio);
 	if (!req) {
-		up_read(&header->snap_rwsem);
+		up_read(&dev->header_rwsem);
 		ret = -ENOMEM;
 		goto done_pages;
 	}
@@ -946,7 +945,7 @@ static int rbd_do_request(struct request *rq,
 				snapc,
 				&mtime,
 				req->r_oid, req->r_oid_len);
-	up_read(&header->snap_rwsem);
+	up_read(&dev->header_rwsem);
 
 	if (linger_req) {
 		ceph_osdc_set_request_linger(osdc, req);
@@ -1718,7 +1717,7 @@ static int __rbd_update_snaps(struct rbd_device *rbd_dev)
 	/* resized? */
 	set_capacity(rbd_dev->disk, h.image_size / SECTOR_SIZE);
 
-	down_write(&rbd_dev->header.snap_rwsem);
+	down_write(&rbd_dev->header_rwsem);
 
 	snap_seq = rbd_dev->header.snapc->seq;
 	if (rbd_dev->header.total_snaps &&
@@ -1743,7 +1742,7 @@ static int __rbd_update_snaps(struct rbd_device *rbd_dev)
 
 	ret = __rbd_init_snaps_header(rbd_dev);
 
-	up_write(&rbd_dev->header.snap_rwsem);
+	up_write(&rbd_dev->header_rwsem);
 
 	return ret;
 }
@@ -2380,8 +2379,9 @@ static ssize_t rbd_add(struct bus_type *bus,
 	spin_lock_init(&rbd_dev->lock);
 	INIT_LIST_HEAD(&rbd_dev->node);
 	INIT_LIST_HEAD(&rbd_dev->snaps);
+	init_rwsem(&rbd_dev->header_rwsem);
 
-	init_rwsem(&rbd_dev->header.snap_rwsem);
+	init_rwsem(&rbd_dev->header_rwsem);
 
 	/* generate unique id: find highest unique id, add one */
 	rbd_id_get(rbd_dev);