firewire: sbp2: fix memory leak in sbp2_cancel_orbs or at send error

When an ORB was canceled (Command ORB i.e. SCSI request timed out, or
Management ORB timed out), or there was a send error in the initial
transaction, we missed to drop one of the ORB's references and thus
leaked memory.

Background:
In total, we hold 3 references to each Operation Request Block:
  - 1 during sbp2_scsi_queuecommand() or sbp2_send_management_orb()
    respectively,
  - 1 for the duration of the write transaction to the ORB_Pointer or
    Management_Agent register of the target,
  - 1 for as long as the ORB stays within the lu->orb_list, until
    the ORB is unlinked from the list and the orb->callback was
    executed.

The latter one of these 3 references is finished
  - normally by sbp2_status_write() when the target wrote status
    for a pending ORB,
  - or by sbp2_cancel_orbs() in case of an ORB time-out,
  - or by complete_transaction() in case of a send error.
Of them, the latter two lacked the kref_put.

Add the missing kref_put()s.  Add comments to the gets and puts of
references for transaction callbacks and ORB callbacks so that it is
easier to see what is supposed to happen.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/sbp2.c

@@ -450,7 +450,7 @@ static void sbp2_status_write(struct fw_card *card, struct fw_request *request,
 
 	if (&orb->link != &lu->orb_list) {
 		orb->callback(orb, &status);
-		kref_put(&orb->kref, free_orb);
+		kref_put(&orb->kref, free_orb); /* orb callback reference */
 	} else {
 		fw_error("status write for unknown orb\n");
 	}
@@ -480,12 +480,14 @@ static void complete_transaction(struct fw_card *card, int rcode,
 	if (orb->rcode != RCODE_COMPLETE) {
 		list_del(&orb->link);
 		spin_unlock_irqrestore(&card->lock, flags);
+
 		orb->callback(orb, NULL);
+		kref_put(&orb->kref, free_orb); /* orb callback reference */
 	} else {
 		spin_unlock_irqrestore(&card->lock, flags);
 	}
 
-	kref_put(&orb->kref, free_orb);
+	kref_put(&orb->kref, free_orb); /* transaction callback reference */
 }
 
 static void sbp2_send_orb(struct sbp2_orb *orb, struct sbp2_logical_unit *lu,
@@ -501,9 +503,8 @@ static void sbp2_send_orb(struct sbp2_orb *orb, struct sbp2_logical_unit *lu,
 	list_add_tail(&orb->link, &lu->orb_list);
 	spin_unlock_irqrestore(&device->card->lock, flags);
 
-	/* Take a ref for the orb list and for the transaction callback. */
-	kref_get(&orb->kref);
-	kref_get(&orb->kref);
+	kref_get(&orb->kref); /* transaction callback reference */
+	kref_get(&orb->kref); /* orb callback reference */
 
 	fw_send_request(device->card, &orb->t, TCODE_WRITE_BLOCK_REQUEST,
 			node_id, generation, device->max_speed, offset,
@@ -530,6 +531,7 @@ static int sbp2_cancel_orbs(struct sbp2_logical_unit *lu)
 
 		orb->rcode = RCODE_CANCELLED;
 		orb->callback(orb, NULL);
+		kref_put(&orb->kref, free_orb); /* orb callback reference */
 	}
 
 	return retval;