net/smc: reduce unnecessary blocking in smcr_lgr_reg_rmbs()

Unlike smc_buf_create() and smcr_buf_unuse(), smcr_lgr_reg_rmbs() is
exclusive when assigned rmb_desc was not registered, although it can be
executed in parallel when assigned rmb_desc was registered already
and only performs read semtamics on it. Hence, we can not simply replace
it with read semaphore.

The idea here is that if the assigned rmb_desc was registered already,
use read semaphore to protect the critical section, once the assigned
rmb_desc was not registered, keep using keep write semaphore still
to keep its exclusivity.

Thanks to the reusable features of rmb_desc, which allows us to execute
in parallel in most cases.
Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/smc/af_smc.c

@@ -519,11 +519,26 @@ static int smcr_lgr_reg_rmbs(struct smc_link *link,
 			     struct smc_buf_desc *rmb_desc)
 {
 	struct smc_link_group *lgr = link->lgr;
+	bool do_slow = false;
 	int i, rc = 0;
 
 	rc = smc_llc_flow_initiate(lgr, SMC_LLC_FLOW_RKEY);
 	if (rc)
 		return rc;
+
+	down_read(&lgr->llc_conf_mutex);
+	for (i = 0; i < SMC_LINKS_PER_LGR_MAX; i++) {
+		if (!smc_link_active(&lgr->lnk[i]))
+			continue;
+		if (!rmb_desc->is_reg_mr[link->link_idx]) {
+			up_read(&lgr->llc_conf_mutex);
+			goto slow_path;
+		}
+	}
+	/* mr register already */
+	goto fast_path;
+slow_path:
+	do_slow = true;
 	/* protect against parallel smc_llc_cli_rkey_exchange() and
 	 * parallel smcr_link_reg_buf()
 	 */
@@ -535,7 +550,7 @@ static int smcr_lgr_reg_rmbs(struct smc_link *link,
 		if (rc)
 			goto out;
 	}
-
+fast_path:
 	/* exchange confirm_rkey msg with peer */
 	rc = smc_llc_do_confirm_rkey(link, rmb_desc);
 	if (rc) {
@@ -544,7 +559,7 @@ static int smcr_lgr_reg_rmbs(struct smc_link *link,
 	}
 	rmb_desc->is_conf_rkey = true;
 out:
-	up_write(&lgr->llc_conf_mutex);
+	do_slow ? up_write(&lgr->llc_conf_mutex) : up_read(&lgr->llc_conf_mutex);
 	smc_llc_flow_stop(lgr, &lgr->llc_flow_lcl);
 	return rc;
 }