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

Lino Sanfilippo
2016-05-22T11:30:27Z

On 22.05.2016 11:17, Shuyu Wei wrote:

> Hi Lino,
>
> I tested this patch, it still got panic under stress.
> Just wget 2 large files simultaneously and it failed.
>
> Looks like the problem comes from the if statement in tx_clean().
> I changed your patch to use
>
> - if (info & FOR_EMAC)
> + if ((info & FOR_EMAC) || !txbd->data || !skb)
>
> and it worked.

Thanks for testing. However that extra check for skb not being NULL should not be
necessary if the code were correct. The changes I suggested were all about having
skb and info consistent with txbd_curr.
But I just realized that there is still a big flaw in the last changes. While
tx() looks correct now (we first set up the descriptor and assign the skb and _then_
advance txbd_curr) tx_clean still is not:

We _first_ have to read tx_curr and _then_ read the corresponding descriptor and its skb.
(The last patch implemented just the reverse - and thus wrong - order, first get skb and
descriptor and then read tx_curr).

So the patch below hopefully handles also tx_clean correctly. Could you please do once more a test
with this one?

>
> After further test, my patch to barrier timestamp() didn't work.
> Just like the original code in the tree, the emac still got stuck under
> high load, even if I changed the smp_wmb() to dma_wmb(). So the original
> code do have race somewhere.

So to make this clear: with the current code in net-next you still see a problem (lockup), right?

>
> I'm new to kernel development, and still trying to understand how memory
> barrier works

Its an interresting topic and thats what I am trying to understand, too :)

> ... and why Francois' fix worked. Please be patient with me :-).

So which fix(es) exactly work for you and solve your lockup issue?

--- 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;

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

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

- /* 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();

- 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();

- 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);
- }

arc_reg_set(priv, R_STATUS, TXPL_MASK);

--
1.9.1