Re: [PATCH v3 06/15] NTB: ntb_transport: Avoid losing QP link-up requests

[Dave Jiang]

On 9/28/26 8:25 AM, 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.
> 
> 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>

With Logan's comment addressed,
Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>

Maybe Jon can amend it on apply.


> ---
> Changes in v3:
>   - Drop the *_ONCE changes. client_ready is now atomic_t.
> 
> v2: https://lore.kernel.org/r/20260910040836.3792333-6-den@xxxxxxxxxxxxx/
> 
>  drivers/ntb/ntb_transport.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
> index 51d9e9969065..d290e5869c21 100644
> --- a/drivers/ntb/ntb_transport.c
> +++ b/drivers/ntb/ntb_transport.c
> @@ -1101,6 +1101,12 @@ static void ntb_transport_link_work(struct work_struct *work)
>  	/* Publish the link only after every QP has been set up. */
>  	atomic_set_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];
>  
> @@ -2400,6 +2406,9 @@ void ntb_transport_link_up(struct ntb_transport_qp *qp)
>  
>  	atomic_set(&qp->client_ready, true);
>  
> +	/* Pairs with the barrier in ntb_transport_link_work(). */
> +	smp_mb();
> +
>  	ntb_transport_schedule_qp_link(qp, 0);
>  }
>  EXPORT_SYMBOL_GPL(ntb_transport_link_up);