All of lore.kernel.org
 help / color / mirror / Atom feed
From: Jakub Kicinski <kuba@kernel.org>
To: Robert Hancock <robert.hancock@calian.com>
Cc: netdev@vger.kernel.org, radhey.shyam.pandey@xilinx.com,
	davem@davemloft.net, edumazet@google.com, pabeni@redhat.com,
	michal.simek@xilinx.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH net-next v6 1/2] net: axienet: Be more careful about updating tx_bd_tail
Date: Wed, 11 May 2022 13:00:28 -0700	[thread overview]
Message-ID: <20220511130028.3e2edb35@kernel.org> (raw)
In-Reply-To: <20220511184432.1131256-2-robert.hancock@calian.com>

On Wed, 11 May 2022 12:44:31 -0600 Robert Hancock wrote:
> The axienet_start_xmit function was updating the tx_bd_tail variable
> multiple times, with potential rollbacks on error or invalid
> intermediate positions, even though this variable is also used in the
> TX completion path. Use READ_ONCE and WRITE_ONCE to make this update
> more atomic, and move the write before the MMIO write to start the
> transfer, so it is protected by that implicit write barrier.
> 
> Signed-off-by: Robert Hancock <robert.hancock@calian.com>
> ---
>  .../net/ethernet/xilinx/xilinx_axienet_main.c | 23 +++++++++++--------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index d6fc3f7acdf0..2f39eb4de249 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -807,12 +807,15 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>  	u32 csum_index_off;
>  	skb_frag_t *frag;
>  	dma_addr_t tail_p, phys;
> +	u32 orig_tail_ptr, new_tail_ptr;
>  	struct axienet_local *lp = netdev_priv(ndev);
>  	struct axidma_bd *cur_p;
> -	u32 orig_tail_ptr = lp->tx_bd_tail;
> +
> +	orig_tail_ptr = READ_ONCE(lp->tx_bd_tail);

This one does not need READ_ONCE().

We only need to protect reads and writes which may race with each other.
This read can't race with any write. We need WRITE_ONCE() in
axienet_start_xmit() and READ_ONCE() in xienet_check_tx_bd_space().

BTW I'm slightly murky on what the rmb() in xienet_check_tx_bd_space()
does. Memory barrier is a fence, not a flush, I don't see what two
accesses that rmb() is separating.

> +	new_tail_ptr = orig_tail_ptr;
>  
>  	num_frag = skb_shinfo(skb)->nr_frags;
> -	cur_p = &lp->tx_bd_v[lp->tx_bd_tail];
> +	cur_p = &lp->tx_bd_v[orig_tail_ptr];
>  
>  	if (axienet_check_tx_bd_space(lp, num_frag + 1)) {
>  		/* Should not happen as last start_xmit call should have


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

WARNING: multiple messages have this Message-ID (diff)
From: Jakub Kicinski <kuba@kernel.org>
To: Robert Hancock <robert.hancock@calian.com>
Cc: netdev@vger.kernel.org, radhey.shyam.pandey@xilinx.com,
	davem@davemloft.net, edumazet@google.com, pabeni@redhat.com,
	michal.simek@xilinx.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH net-next v6 1/2] net: axienet: Be more careful about updating tx_bd_tail
Date: Wed, 11 May 2022 13:00:28 -0700	[thread overview]
Message-ID: <20220511130028.3e2edb35@kernel.org> (raw)
In-Reply-To: <20220511184432.1131256-2-robert.hancock@calian.com>

On Wed, 11 May 2022 12:44:31 -0600 Robert Hancock wrote:
> The axienet_start_xmit function was updating the tx_bd_tail variable
> multiple times, with potential rollbacks on error or invalid
> intermediate positions, even though this variable is also used in the
> TX completion path. Use READ_ONCE and WRITE_ONCE to make this update
> more atomic, and move the write before the MMIO write to start the
> transfer, so it is protected by that implicit write barrier.
> 
> Signed-off-by: Robert Hancock <robert.hancock@calian.com>
> ---
>  .../net/ethernet/xilinx/xilinx_axienet_main.c | 23 +++++++++++--------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index d6fc3f7acdf0..2f39eb4de249 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -807,12 +807,15 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>  	u32 csum_index_off;
>  	skb_frag_t *frag;
>  	dma_addr_t tail_p, phys;
> +	u32 orig_tail_ptr, new_tail_ptr;
>  	struct axienet_local *lp = netdev_priv(ndev);
>  	struct axidma_bd *cur_p;
> -	u32 orig_tail_ptr = lp->tx_bd_tail;
> +
> +	orig_tail_ptr = READ_ONCE(lp->tx_bd_tail);

This one does not need READ_ONCE().

We only need to protect reads and writes which may race with each other.
This read can't race with any write. We need WRITE_ONCE() in
axienet_start_xmit() and READ_ONCE() in xienet_check_tx_bd_space().

BTW I'm slightly murky on what the rmb() in xienet_check_tx_bd_space()
does. Memory barrier is a fence, not a flush, I don't see what two
accesses that rmb() is separating.

> +	new_tail_ptr = orig_tail_ptr;
>  
>  	num_frag = skb_shinfo(skb)->nr_frags;
> -	cur_p = &lp->tx_bd_v[lp->tx_bd_tail];
> +	cur_p = &lp->tx_bd_v[orig_tail_ptr];
>  
>  	if (axienet_check_tx_bd_space(lp, num_frag + 1)) {
>  		/* Should not happen as last start_xmit call should have


  reply	other threads:[~2022-05-11 20:01 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-11 18:44 [PATCH net-next v6 0/2] axienet NAPI improvements Robert Hancock
2022-05-11 18:44 ` Robert Hancock
2022-05-11 18:44 ` [PATCH net-next v6 1/2] net: axienet: Be more careful about updating tx_bd_tail Robert Hancock
2022-05-11 18:44   ` Robert Hancock
2022-05-11 20:00   ` Jakub Kicinski [this message]
2022-05-11 20:00     ` Jakub Kicinski
2022-05-11 20:40     ` Robert Hancock
2022-05-11 20:40       ` Robert Hancock
2022-05-11 18:44 ` [PATCH net-next v6 2/2] net: axienet: Use NAPI for TX completion path Robert Hancock
2022-05-11 18:44   ` Robert Hancock

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20220511130028.3e2edb35@kernel.org \
    --to=kuba@kernel.org \
    --cc=davem@davemloft.net \
    --cc=edumazet@google.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=michal.simek@xilinx.com \
    --cc=netdev@vger.kernel.org \
    --cc=pabeni@redhat.com \
    --cc=radhey.shyam.pandey@xilinx.com \
    --cc=robert.hancock@calian.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.