drbd: protect updates to integrits_tfm by tconn->data->mutex

Since we need to hold that mutex anyways to make sure the peer
gets that change in the right position in the data stream,
it makes a lot of sense to use the same mutex to ensure existence
of the tfm.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_int.h

@@ -848,7 +848,7 @@ struct drbd_tconn {			/* is a resource from the config file */
 	struct list_head out_of_sequence_requests;
 
 	struct crypto_hash *cram_hmac_tfm;
-	struct crypto_hash *integrity_tfm;  /* checksums we compute */
+	struct crypto_hash *integrity_tfm;  /* checksums we compute, updates protected by tconn->data->mutex */
 	struct crypto_hash *peer_integrity_tfm;  /* checksums we verify */
 	struct crypto_hash *csums_tfm;
 	struct crypto_hash *verify_tfm;

drivers/block/drbd/drbd_main.c

@@ -1720,11 +1720,11 @@ int drbd_send_dblock(struct drbd_conf *mdev, struct drbd_request *req)
 	int dgs;
 	int err;
 
+	sock = &mdev->tconn->data;
+	p = drbd_prepare_command(mdev, sock);
 	dgs = (mdev->tconn->agreed_pro_version >= 87 && mdev->tconn->integrity_tfm) ?
 		crypto_hash_digestsize(mdev->tconn->integrity_tfm) : 0;
 
-	sock = &mdev->tconn->data;
-	p = drbd_prepare_command(mdev, sock);
 	if (!p)
 		return -EIO;
 	p->sector = cpu_to_be64(req->i.sector);
@@ -1793,11 +1793,12 @@ int drbd_send_block(struct drbd_conf *mdev, enum drbd_packet cmd,
 	int err;
 	int dgs;
 
+	sock = &mdev->tconn->data;
+	p = drbd_prepare_command(mdev, sock);
+
 	dgs = (mdev->tconn->agreed_pro_version >= 87 && mdev->tconn->integrity_tfm) ?
 		crypto_hash_digestsize(mdev->tconn->integrity_tfm) : 0;
 
-	sock = &mdev->tconn->data;
-	p = drbd_prepare_command(mdev, sock);
 	if (!p)
 		return -EIO;
 	p->sector = cpu_to_be64(peer_req->i.sector);