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

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Date: Thu, 16 Nov 2023 07:02:50 +0000
From: "Li,Rongqing" <lirongqing@...du.com>
To: "dust.li@...ux.alibaba.com" <dust.li@...ux.alibaba.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-s390@...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

> -----Original Message-----
> From: Dust Li <dust.li@...ux.alibaba.com>
> Sent: Thursday, November 16, 2023 2:18 PM
> 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".
> 

Ok, I will fix it, thanks

-Li


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