netdev - RE: [PATCH net 1/1] qede: Fix barrier usage after tx doorbell write.

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Date:   Wed, 21 Mar 2018 13:39:37 +0000
From:   "Elior, Ariel" <Ariel.Elior@...ium.com>
To:     "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Kalderon, Michal" <Michal.Kalderon@...ium.com>,
        "Chopra, Manish" <Manish.Chopra@...ium.com>
Subject: RE: [PATCH net 1/1] qede: Fix barrier usage after tx doorbell write.

> Subject: [PATCH net 1/1] qede: Fix barrier usage after tx doorbell write.
> 
> Since commit c5ad119fb6c09b0297446be05bd66602fa564758
> ("net: sched: pfifo_fast use skb_array") driver is exposed
> to an issue where it is hitting NULL skbs while handling TX
> completions. Driver uses mmiowb() to flush the writes to the
> doorbell bar which is a write-combined bar, however on x86
> mmiowb() does not flush the write combined buffer.
> 
> This patch fixes this problem by replacing mmiowb() with wmb()
> after the write combined doorbell write so that writes are
> flushed and synchronized from more than one processor.
> 
> Signed-off-by: Ariel Elior <ariel.elior@...ium.com>
> Signed-off-by: Manish Chopra <manish.chopra@...ium.com>
> ---
>  drivers/net/ethernet/qlogic/qede/qede_fp.c |   10 ++++------
>  1 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/qlogic/qede/qede_fp.c
> b/drivers/net/ethernet/qlogic/qede/qede_fp.c
> index dafc079..2e921ca 100644
> --- a/drivers/net/ethernet/qlogic/qede/qede_fp.c
> +++ b/drivers/net/ethernet/qlogic/qede/qede_fp.c
> @@ -320,13 +320,11 @@ static inline void qede_update_tx_producer(struct
> qede_tx_queue *txq)
>  	barrier();
>  	writel(txq->tx_db.raw, txq->doorbell_addr);
> 
> -	/* mmiowb is needed to synchronize doorbell writes from more than one
> -	 * processor. It guarantees that the write arrives to the device before
> -	 * the queue lock is released and another start_xmit is called (possibly
> -	 * on another CPU). Without this barrier, the next doorbell can bypass
> -	 * this doorbell. This is applicable to IA64/Altix systems.
> +	/* Fence required to flush the write combined buffer, since another
> +	 * CPU may write to the same doorbell address and data may be lost
> +	 * due to relaxed order nature of write combined bar.
>  	 */
> -	mmiowb();
> +	wmb();
>  }
> 
>  static int qede_xdp_xmit(struct qede_dev *edev, struct qede_fastpath *fp,
> --
> 1.7.1

Hi Dave,
This patch appears as "superseded" in patchwork. I am not really sure why
that is - I noticed some other barrier work is going on, but none of it will
solve this issue. This patch solves an important bug in the driver - please
consider applying it.
Thanks,
Ariel