On 17.11.23 12:16, 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
>
> Reviewed-by: Dust Li <dust.li@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> ---
> 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;
> }
It seems to me that the purpose of conn->tx_pushing is
a) Serve as a mutex, so only one thread per conn will call __smc_tx_sndbuf_nonempty().
b) Repeat, in case some other thread has added data to sndbuf concurrently.
I agree that this patch does not change the behaviour of this function and removes an
atomic_set() in the likely path.
I wonder however: All callers of smc_tx_sndbuf_nonempty() must hold the socket lock.
So how can we ever run in a concurrency situation?
Is this handling of conn->tx_pushing necessary at all?
