On Fri, Nov 17, 2023 at 01:27:57PM +0100, Alexandra Winter wrote:
>
>
>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?
Hi Sandy,
Overall, I think you are right. But there is something we need to take care.
Before commit 6b88af839d20 ("net/smc: don't send in the BH context if
sock_owned_by_user"), we used to call smc_tx_pending() in the soft IRQ,
without checking sock_owned_by_user(), which would caused a race condition
because bh_lock_sock() did not honor sock_lock(). To address this issue,
I have added the tx_pushing mechanism. However, with commit 6b88af839d20,
we now defer the transmission if sock_lock() is held by the user.
Therefore, there should no longer be a race condition. Nevertheless, if
we remove the tx_pending mechanism, we must always remember not to call
smc_tx_sndbuf_nonempty() in the soft IRQ when the user holds the sock lock.
Thanks
Dust
