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

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Message-ID: <20231116061811.GC121324@linux.alibaba.com>
Date: Thu, 16 Nov 2023 14:18:11 +0800
From: Dust Li <dust.li@...ux.alibaba.com>
To: Li RongQing <lirongqing@...du.com>, wenjia@...ux.ibm.co,
	netdev@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH][net-next] net/smc: avoid atomic_set and smp_wmb in the
 tx path when possible

On Thu, Nov 16, 2023 at 10:20:41AM +0800, Li RongQing wrote:
>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
>
>Signed-off-by: Li RongQing <lirongqing@...du.com>
Reviewed-by: Dust Li <dust.li@...ux.alibaba.com>

>---
> 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)
> 		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 real send */
nit: it would be better if we change the comments to "send again".

Thanks
> 		goto again;
>+	}
> 
> 	return rc;
> }
>-- 
>2.9.4