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

Lino Sanfilippo
2016-05-24T01:09:49Z

On 23.05.2016 00:36, Francois Romieu wrote:
> Lino Sanfilippo <LinoSanfilippo-Mmb7MZpHnFY@public.gmane.org> :
> [...]
>> --- 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 Insert a smp_rmb() here to close one window for an outdated txbd_dirty value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.

I agree, we should place the barrier before the loop. I also think it is indeed more
appropriate to use the SMP versions for both barriers.

>
> 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.

I cant deny that :)

>
>>
>> + /* 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().

Ok.

>> +
>> + 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 dont agree here. A dma_wmb would have an effect to "data" and "info", yes, but it
would have absolutely no effect to skb_tx_timestamp(), since there is no dma access
involved here. In fact skb_tx_timestamp() could probably be even reordered to appear
after the dma_wmb.
Anyway, there is the wmb() directly after the assignment to "info". _This_ barrier
should ensure that skb_tx_timestamp() (along with a flush of data and info to DMA)
is executed before "txbd_curr" is advanced.
This means that the corresponding skb cant be freed prematurely by tx_clean().

>
> [*] 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().
>

I really prefer one generic barrier over combos of 2 or more special barriers.

>>
>> - 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).
>

Ok, I can see the race now. So yes, this should be kept.

I will prepare a patch with the discussed changes tomorrow.

Regards,
Lino