netdev - Re: [PATCH v2] ethernet:arc: Fix racing of TX ring buffer

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Message-ID: <20160522223659.GB5086@electric-eye.fr.zoreil.com>
Date:	Mon, 23 May 2016 00:36:59 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Lino Sanfilippo <LinoSanfilippo@....de>
Cc:	Shuyu Wei <wsy2220@...il.com>, David Miller <davem@...emloft.net>,
	wxt@...k-chips.com, heiko@...ech.de,
	linux-rockchip@...ts.infradead.org, netdev@...r.kernel.org,
	al.kochet@...il.com
Subject: Re: [PATCH v2] ethernet:arc: Fix racing of TX ring buffer

Lino Sanfilippo <LinoSanfilippo@....de> :
[...]
> --- a/drivers/net/ethernet/arc/emac_main.c
> +++ b/drivers/net/ethernet/arc/emac_main.c
> @@ -159,12 +159,22 @@ static void arc_emac_tx_clean(struct net_device *ndev)
>  		unsigned int *txbd_dirty = &priv->txbd_dirty;
>  		struct arc_emac_bd *txbd = &priv->txbd[*txbd_dirty];
>  		struct buffer_state *tx_buff = &priv->tx_buff[*txbd_dirty];
> -		struct sk_buff *skb = tx_buff->skb;
>  		unsigned int info = le32_to_cpu(txbd->info);
> +		struct sk_buff *skb;
>  

Insert a smp_rmb() here to close one window for an outdated txbd_dirty value
(the "arc_emac_tx_clean wrote txbd_curr and issued smp_wmb" one).

Actually, insert smp_rmb() at the start of arc_emac_tx_clean() as it
does not need to be performed withing the loop and would penalize it.

Given the implicit smp barriers in the non-driver code, I consider
"arc_emac_tx_clean on one CPU does not read latest txbd_dirty value written
by previous arc_emac_tx_clean on different CPU" as utter onanism but
issueing smp_rmb() at the start of arc_emac_tx_clean() nails it as well.

> -		if ((info & FOR_EMAC) || !txbd->data || !skb)
> +		if (*txbd_dirty == priv->txbd_curr)
>  			break;

Ok, it's just the "while (priv->txbd_dirty != priv->txbd_curr) {" loop
in disguise.

>  
> +		/* Make sure curr pointer is consistent with info */
> +		rmb();
> +
> +		info = le32_to_cpu(txbd->info);
> +
> +		if (info & FOR_EMAC)
> +			break;

With proper ordering + barrier in arc_emac_tx() you can relax it to smp_rmb().

> +
> +		skb = tx_buff->skb;
> +
>  		if (unlikely(info & (DROP | DEFR | LTCL | UFLO))) {
>  			stats->tx_errors++;
>  			stats->tx_dropped++;
> @@ -195,8 +205,8 @@ static void arc_emac_tx_clean(struct net_device *ndev)
>  		*txbd_dirty = (*txbd_dirty + 1) % TX_BD_NUM;
>  	}
>  
> -	/* Ensure that txbd_dirty is visible to tx() before checking
> -	 * for queue stopped.
> +	/* Ensure that txbd_dirty is visible to tx() and we see the most recent
> +	 * value for txbd_curr.
>  	 */
>  	smp_mb();
>  
> @@ -680,35 +690,29 @@ static int arc_emac_tx(struct sk_buff *skb, struct net_device *ndev)
>  	dma_unmap_len_set(&priv->tx_buff[*txbd_curr], len, len);
>  
>  	priv->txbd[*txbd_curr].data = cpu_to_le32(addr);
> -
> -	/* Make sure pointer to data buffer is set */
> -	wmb();
> +	priv->tx_buff[*txbd_curr].skb = skb;
>  
>  	skb_tx_timestamp(skb);
>  
>  	*info = cpu_to_le32(FOR_EMAC | FIRST_OR_LAST_MASK | len);

No.

You need dma_wmb() after skb_tx_timestamp() to commit skb_tx_timestamp() [*]
and data = cpu_to_le32(addr).

[*] I doubt anyone want a dma_sync_single_...() here.

>  
> -	/* Make sure info word is set */
> +	/* 1. Make sure that with respect to tx_clean everything is set up
> +	 * properly before we advance txbd_curr.
> +	 * 2. Make sure writes to DMA descriptors are completed before we inform
> +	 * the hardware.
> +	 */
>  	wmb();

Yes, either wmb() or smp_wmb() + dma_wmb().

>  
> -	priv->tx_buff[*txbd_curr].skb = skb;
> -
>  	/* Increment index to point to the next BD */
>  	*txbd_curr = (*txbd_curr + 1) % TX_BD_NUM;
>  
> -	/* Ensure that tx_clean() sees the new txbd_curr before
> -	 * checking the queue status. This prevents an unneeded wake
> -	 * of the queue in tx_clean().
> +	/* Ensure we see the most recent value of txbd_dirty and tx_clean() sees
> +	 * the updated value of txbd_curr.
>  	 */
>  	smp_mb();

Nit: s/the most/a/

"a" as in "arc_emac_tx_clean() _is_ racing with arc_emac_tx"

>  
> -	if (!arc_emac_tx_avail(priv)) {
> +	if (!arc_emac_tx_avail(priv))
>  		netif_stop_queue(ndev);
> -		/* Refresh tx_dirty */
> -		smp_mb();
> -		if (arc_emac_tx_avail(priv))
> -			netif_start_queue(ndev);
> -	}

No.

I may sound like an old record but the revalidation part must be kept.

txbd_dirty may change in the arc_emac_tx_avail.. netif_stop_queue window
(the race requires a different CPU as arc_emac_tx() runs in locally
disabled BH context).

-- 
Ueimor