Re: [PATCH net-next v2 1/4] NTB: ntb_transport: Order RX descriptor reads after completion

On 8/16/26 11:49 PM, Koichiro Den wrote:
> The peer writes payloads and descriptors into a DMA-coherent memory
> window. ntb_process_rxc() checks DESC_DONE_FLAG before consuming the
> descriptor and payload, but coherent memory alone does not order those
> reads on weakly ordered CPUs.
> 
> Read the completion word once and issue dma_rmb() after DONE is observed.
> Use the saved word for subsequent transport flag checks.
> 
> Fixes: fce8a7bb5b4b ("PCI-Express Non-Transparent Bridge Support")
> Reported-by: Sashiko <[email protected]>
> Link: https://lore.kernel.org/r/[email protected]/
> Signed-off-by: Koichiro Den <[email protected]>

Reviewed-by: Dave Jiang <[email protected]>

> ---
> Changes in v2:
>   - New patch. (Sashiko)
> 
>  drivers/ntb/ntb_transport.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
> index f59f926d4bfa..d458a8b1de11 100644
> --- a/drivers/ntb/ntb_transport.c
> +++ b/drivers/ntb/ntb_transport.c
> @@ -1609,21 +1609,25 @@ static int ntb_process_rxc(struct ntb_transport_qp *qp)
>  {
>  	struct ntb_payload_header *hdr;
>  	struct ntb_queue_entry *entry;
> +	unsigned int flags;
>  	void *offset;
>  
>  	offset = qp->rx_buff + qp->rx_max_frame * qp->rx_index;
>  	hdr = offset + qp->rx_max_frame - sizeof(struct ntb_payload_header);
>  
> -	dev_dbg(&qp->ndev->pdev->dev, "qp %d: RX ver %u len %d flags %x\n",
> -		qp->qp_num, hdr->ver, hdr->len, hdr->flags);
> -
> -	if (!(hdr->flags & DESC_DONE_FLAG)) {
> +	flags = READ_ONCE(hdr->flags);
> +	if (!(flags & DESC_DONE_FLAG)) {
>  		dev_dbg(&qp->ndev->pdev->dev, "done flag not set\n");
>  		qp->rx_ring_empty++;
>  		return -EAGAIN;
>  	}
>  
> -	if (hdr->flags & LINK_DOWN_FLAG) {
> +	dma_rmb();
> +
> +	dev_dbg(&qp->ndev->pdev->dev, "qp %d: RX ver %u len %d flags %x\n",
> +		qp->qp_num, hdr->ver, hdr->len, flags);
> +
> +	if (flags & LINK_DOWN_FLAG) {
>  		dev_dbg(&qp->ndev->pdev->dev, "link down flag set\n");
>  		ntb_qp_link_down(qp);
>  		hdr->flags = 0;