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

Shuyu Wei
2016-05-23T11:36:09Z

On Sun, May 22, 2016 at 01:30:27PM +0200, Lino Sanfilippo wrote:
>
> 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?

Hi Lino,
This patch worked after a whole night of stress testing.

> >
> > 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?
Yes, I mean the mainline kernel, which should be the same as net-next.

> > ... and why Francois' fix worked. Please be patient with me :-).
>
> So which fix(es) exactly work for you and solve your lockup issue?
I mean the patch below, starting this thread.

diff --git a/drivers/net/ethernet/arc/emac_main.c b/drivers/net/ethernet/arc/emac_main.c
index a3a9392..df3dfef 100644
--- a/drivers/net/ethernet/arc/emac_main.c
+++ b/drivers/net/ethernet/arc/emac_main.c
@@ -153,9 +153,8 @@ static void arc_emac_tx_clean(struct net_device *ndev)
{
struct arc_emac_priv *priv = netdev_priv(ndev);
struct net_device_stats *stats = &ndev->stats;
- unsigned int i;

- for (i = 0; i < TX_BD_NUM; i++) {
+ while (priv->txbd_dirty != priv->txbd_curr) {
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];
@@ -685,13 +684,15 @@ static int arc_emac_tx(struct sk_buff *skb, struct net_device *ndev)
wmb();

skb_tx_timestamp(skb);
+ priv->tx_buff[*txbd_curr].skb = skb;
+
+ dma_wmb();

*info = cpu_to_le32(FOR_EMAC | FIRST_OR_LAST_MASK | len);

/* Make sure info word is set */
wmb();

- priv->tx_buff[*txbd_curr].skb = skb;

/* Increment index to point to the next BD */
*txbd_curr = (*txbd_curr + 1) % TX_BD_NUM;