Re: [PATCH net-next v4 2/3] net/smc: reduce TX slot contention with exclusive wait

On 2026/7/16 19:37, D. Wythe wrote:
> smc_wr_tx_get_free_slot() waits for a free TX slot with
> wait_event_interruptible_timeout(). Since the wait_event family
> enqueues waiters as non-exclusive, wake_up() may wake multiple
> waiters even though only one can use the slot, causing
> thundering-herd contention when slots are scarce.
> 
> Use an exclusive wait loop with prepare_to_wait_exclusive() so
> wake_up() wakes only one waiter per freed slot.
> smc_wr_wakeup_tx_wait() still uses wake_up_all() during link
> teardown, so teardown behavior is unchanged.
> 
> This also corrects the return value on a pending signal: the previous
> wait_event_interruptible_timeout() path fell through to the "no free
> slot" case and returned -EPIPE, masking the signal as a connection
> error. The open-coded loop now returns -ERESTARTSYS, matching the
> standard interruptible-wait semantics and letting the syscall restart
> machinery handle it.
> 
> Signed-off-by: D. Wythe <[email protected]>
> ---

Reviewed-by: Wen Gu <[email protected]>

Thanks.

>   net/smc/smc_wr.c | 36 ++++++++++++++++++++++++++----------
>   1 file changed, 26 insertions(+), 10 deletions(-)
> 
> diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c
> index 6b5add922993..2cdd4063c13d 100644
> --- a/net/smc/smc_wr.c
> +++ b/net/smc/smc_wr.c
> @@ -198,11 +198,13 @@ int smc_wr_tx_get_free_slot(struct smc_link *link,
>   			    struct smc_rdma_wr **wr_rdma_buf,
>   			    struct smc_wr_tx_pend_priv **wr_pend_priv)
>   {
> +	unsigned long timeout = SMC_WR_TX_WAIT_FREE_SLOT_TIME;
>   	struct smc_link_group *lgr = smc_get_lgr(link);
>   	struct smc_wr_tx_pend *wr_pend;
>   	u32 idx = link->wr_tx_cnt;
>   	struct ib_send_wr *wr_ib;
>   	u64 wr_id;
> +	DEFINE_WAIT(wait);
>   	int rc;
>   
>   	*wr_buf = NULL;
> @@ -212,17 +214,31 @@ int smc_wr_tx_get_free_slot(struct smc_link *link,
>   		if (rc)
>   			return rc;
>   	} else {
> -		rc = wait_event_interruptible_timeout(
> -			link->wr_tx_wait,
> -			!smc_link_sendable(link) ||
> -			lgr->terminating ||
> -			(smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY),
> -			SMC_WR_TX_WAIT_FREE_SLOT_TIME);
> -		if (!rc) {
> -			/* timeout - terminate link */
> -			smcr_link_down_cond_sched(link);
> -			return -EPIPE;
> +		rc = 0;
> +		for (;;) {
> +			prepare_to_wait_exclusive(&link->wr_tx_wait, &wait,
> +						  TASK_INTERRUPTIBLE);
> +			if (!smc_link_sendable(link) || lgr->terminating ||
> +			    smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY)
> +				break;
> +			timeout = schedule_timeout(timeout);
> +			/* re-check */
> +			if (!smc_link_sendable(link) || lgr->terminating ||
> +			    smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY)
> +				break;
> +			if (!timeout) {
> +				/* timeout - terminate link */
> +				smcr_link_down_cond_sched(link);
> +				break;
> +			}
> +			if (signal_pending(current)) {
> +				rc = -ERESTARTSYS;
> +				break;
> +			}
>   		}
> +		finish_wait(&link->wr_tx_wait, &wait);
> +		if (rc)
> +			return rc;
>   		if (idx == link->wr_tx_cnt)
>   			return -EPIPE;
>   	}