RE: [PATCH] net/smc: avoid atomic_set and smp_wmb in the tx path when possible

-----Original Message-----
From: Wenjia Zhang <redacted>
Sent: Friday, November 3, 2023 4:42 AM
To: Li,Rongqing <redacted>
Cc: linux-s390@vger.kernel.org; netdev@vger.kernel.org
Subject: Re: [PATCH] net/smc: avoid atomic_set and smp_wmb in the tx path
when possible



On 02.11.23 10:27, Li RongQing wrote:

quoted

these is less opportunity 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 when possible

I think we should avoid to use argument like "less opportunity" in commit
message. Because "less opportunity" does not mean "no opportunity". Once it
occurs, does it mean that what the patch changes is useless or wrong?

I will reword the message.
I think this is a question of probability. even tx_pushing is not 1, this is still not a problem, atomic_dec_and_test(&conn->tx_pushing) will return false, transmit will be looped again, and tx_pushing will be added at any time

quoted

Signed-off-by: Li RongQing <redacted>
---
  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..72dbdee 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)

quoted

  		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

quoted

@@ -677,8 +675,11 @@ int smc_tx_sndbuf_nonempty(struct

smc_connection *conn)

quoted

  	 * 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 real send */
  		goto again;
+	}

  	return rc;
  }

I'm afraid that the *if* statement would never be true, without setting the
value of &conn->tx_pushing firstly.

I think conn->tx_pushing do not need to be set in this condition, and this patch is trying to avoid setting it 

Thanks

-Li