linux-kernel - Re: [net PATCH] octeon_ep: update BQL sent bytes before ringing doorbell

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Message-ID: <237ad66815a7988eaf9b0ed2132772c58e868cd8.camel@redhat.com>
Date:   Thu, 12 Oct 2023 10:31:33 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Shinas Rasheed <srasheed@...vell.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, hgani@...vell.com
Cc:     vimleshk@...vell.com, egallen@...hat.com, mschmidt@...hat.com,
        Veerasenareddy Burru <vburru@...vell.com>,
        Sathesh Edara <sedara@...vell.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Satananda Burla <sburla@...vell.com>,
        Abhijit Ayarekar <aayarekar@...vell.com>
Subject: Re: [net PATCH] octeon_ep: update BQL sent bytes before ringing
 doorbell

On Tue, 2023-10-10 at 04:50 -0700, Shinas Rasheed wrote:
> Sometimes Tx is completed immediately after doorbell is updated, which
> causes Tx completion routing to update completion bytes before the
> same packet bytes are updated in sent bytes in transmit function, hence
> hitting BUG_ON() in dql_completed(). To avoid this, update BQL
> sent bytes before ringing doorbell.
> 
> Fixes: 37d79d059606 ("octeon_ep: add Tx/Rx processing and interrupt support")
> Signed-off-by: Shinas Rasheed <srasheed@...vell.com>
> ---
>  drivers/net/ethernet/marvell/octeon_ep/octep_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/marvell/octeon_ep/octep_main.c b/drivers/net/ethernet/marvell/octeon_ep/octep_main.c
> index dbc518ff8276..314f9c661f93 100644
> --- a/drivers/net/ethernet/marvell/octeon_ep/octep_main.c
> +++ b/drivers/net/ethernet/marvell/octeon_ep/octep_main.c
> @@ -718,6 +718,7 @@ static netdev_tx_t octep_start_xmit(struct sk_buff *skb,
>  	/* Flush the hw descriptor before writing to doorbell */
>  	wmb();
>  
> +	netdev_tx_sent_queue(iq->netdev_q, skb->len);

If tx completion and start_xmit happen on 2 different CPUs, how do you
ensure that xmit_completion will observe the values written here?

Specifically, don't you need to move netdev_tx_sent_queue() before the
above memory barrier?

Thanks,

Paolo