netdev - [PATCH net-next v3] net/smc: avoid atomic_set and smp

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Message-Id: <20231117111657.16266-1-lirongqing@baidu.com>
Date: Fri, 17 Nov 2023 19:16:57 +0800
From: Li RongQing <lirongqing@...du.com>
To: kgraul@...ux.ibm.com,
	wenjia@...ux.ibm.com,
	jaka@...ux.ibm.com,
	alibuda@...ux.alibaba.com,
	tonylu@...ux.alibaba.co,
	guwen@...ux.alibaba.com,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	linux-s390@...r.kernel.org,
	netdev@...r.kernel.org,
	dust.li@...ux.alibaba.com
Subject: [PATCH net-next v3] net/smc: avoid atomic_set and smp_wmb in the tx path when possible

There is rare possibility that conn->tx_pushing is not 1, since
tx_pushing is just checked with 1, so move the setting tx_pushing
to 1 after atomic_dec_and_test() return false, to avoid atomic_set
and smp_wmb in tx path

Reviewed-by: Dust Li <dust.li@...ux.alibaba.com>
Signed-off-by: Li RongQing <lirongqing@...du.com>
---
diff v3: improvements in the commit body and comments
diff v2: fix a typo in commit body and add net-next subject-prefix
 net/smc/smc_tx.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/smc/smc_tx.c b/net/smc/smc_tx.c
index 3b0ff3b..2c2933f 100644
--- a/net/smc/smc_tx.c
+++ b/net/smc/smc_tx.c
@@ -667,8 +667,6 @@ int smc_tx_sndbuf_nonempty(struct smc_connection *conn)
 		return 0;
 
 again:
-	atomic_set(&conn->tx_pushing, 1);
-	smp_wmb(); /* Make sure tx_pushing is 1 before real send */
 	rc = __smc_tx_sndbuf_nonempty(conn);
 
 	/* We need to check whether someone else have added some data into
@@ -677,8 +675,11 @@ int smc_tx_sndbuf_nonempty(struct smc_connection *conn)
 	 * If so, we need to push again to prevent those data hang in the send
 	 * queue.
 	 */
-	if (unlikely(!atomic_dec_and_test(&conn->tx_pushing)))
+	if (unlikely(!atomic_dec_and_test(&conn->tx_pushing))) {
+		atomic_set(&conn->tx_pushing, 1);
+		smp_wmb(); /* Make sure tx_pushing is 1 before send again */
 		goto again;
+	}
 
 	return rc;
 }
-- 
2.9.4