tcm_fc: Generate TASK_SET_FULL status for response failures

This patch changes ft_queue_status() to set SAM_STAT_TASK_SET_FULL
status upon lport->tt.seq_send( failure, and return -EAGAIN to notify
target-core to attempt to requeue the response.

It also does the same for a fc_frame_alloc() failures, in order to
signal the initiator that it should try to reduce it's current
queue_depth, to lower the number of outstanding I/Os on the wire.

Reported-by: Vasu Dev <[e-mail]>
Reviewed-by: Vasu Dev <[e-mail]>
Cc: Jun Wu <[e-mail]>
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/tcm_fc/tfc_cmd.c

···
 	struct fc_lport *lport;
 	struct fc_exch *ep;
 	size_t len;
+	int rc;
 
 	if (cmd->aborted)
 		return 0;
···
 	len = sizeof(*fcp) + se_cmd->scsi_sense_length;
 	fp = fc_frame_alloc(lport, len);
 	if (!fp) {
-		/* XXX shouldn't just drop it - requeue and retry? */
-		return 0;
+		se_cmd->scsi_status = SAM_STAT_TASK_SET_FULL;
+		return -ENOMEM;
 	}
+
 	fcp = fc_frame_payload_get(fp, len);
 	memset(fcp, 0, len);
 	fcp->resp.fr_status = se_cmd->scsi_status;
···
 	fc_fill_fc_hdr(fp, FC_RCTL_DD_CMD_STATUS, ep->did, ep->sid, FC_TYPE_FCP,
 		       FC_FC_EX_CTX | FC_FC_LAST_SEQ | FC_FC_END_SEQ, 0);
 
-	lport->tt.seq_send(lport, cmd->seq, fp);
+	rc = lport->tt.seq_send(lport, cmd->seq, fp);
+	if (rc) {
+		pr_info_ratelimited("%s: Failed to send response frame %p, "
+				    "xid <0x%x>\n", __func__, fp, ep->xid);
+		/*
+		 * Generate a TASK_SET_FULL status to notify the initiator
+		 * to reduce it's queue_depth after the se_cmd response has
+		 * been re-queued by target-core.
+		 */
+		se_cmd->scsi_status = SAM_STAT_TASK_SET_FULL;
+		return -ENOMEM;
+	}
 	lport->tt.exch_done(cmd->seq);
 	return 0;
 }