Re: [PATCH v2 05/14] NTB: ntb_transport: Avoid losing QP link-up requests

[Frank Li]

On Thu, Sep 10, 2026 at 01:08:27PM +0900, Koichiro Den wrote:
> ntb_netdev_open() can call ntb_transport_link_up() while the transport
> worker is completing setup on another CPU. Concurrent transport setup
> and a client link-up request can both read the other's flag as false and
> leave QP link work unqueued. The QP then stays down until another link
> event or client link-up request.
>
> This is the store-buffering pattern described in
> tools/memory-model/Documentation/recipes.txt ("Store buffering").
>
> Add a full barrier between the store and load on each side, and
> mark the client_ready accesses with READ_ONCE()/WRITE_ONCE().
>
> Fixes: fce8a7bb5b4b ("PCI-Express Non-Transparent Bridge Support")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/20260907144701.702E41F00A3A@xxxxxxxxxxxxxxx/
> Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
> ---
> Changes in v2:
>   - New patch (Sashiko)
>
>  drivers/ntb/ntb_transport.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
> index 75d4a2e021f4..1332d53bcfe7 100644
> --- a/drivers/ntb/ntb_transport.c
> +++ b/drivers/ntb/ntb_transport.c
> @@ -1104,10 +1104,16 @@ static void ntb_transport_link_work(struct work_struct *work)
>  	/* Publish the link only after every QP has been set up. */
>  	smp_store_release(&nt->link_is_up, true);
>
> +	/*
> +	 * Prevent both sides from missing each other's flag. Pairs with
> +	 * the barrier in ntb_transport_link_up().
> +	 */
> +	smp_mb();
> +
>  	for (i = 0; i < nt->qp_count; i++) {
>  		struct ntb_transport_qp *qp = &nt->qp_vec[i];
>
> -		if (qp->client_ready)
> +		if (READ_ONCE(qp->client_ready))

I think it'd better change to use atomic variable for client_ready to avoid
manual handle smp sync.

Frank

>  			ntb_transport_schedule_qp_link(qp, 0);
>  	}
>
> @@ -2401,7 +2407,10 @@ void ntb_transport_link_up(struct ntb_transport_qp *qp)
>  	if (!qp)
>  		return;
>
> -	qp->client_ready = true;
> +	WRITE_ONCE(qp->client_ready, true);
> +
> +	/* Pairs with the barrier in ntb_transport_link_work(). */
> +	smp_mb();
>
>  	ntb_transport_schedule_qp_link(qp, 0);
>  }
> --
> 2.51.0
>