From: Shannon Nelson <shannon.nelson@xxxxxxxxxx>
Date: Wed, 30 Nov 2016 17:25:30 -0800
> Near the bottom of sunvnet_common.c:sunvnet_start_xmit_common(), about
> line 1423, just after the ldc_start_done: label is a curious 'if'
> block
>
> if (unlikely(vnet_tx_dring_avail(dr) < 1)) {
> netif_tx_stop_queue(txq);
> if (vnet_tx_dring_avail(dr) > VNET_TX_WAKEUP_THRESH(dr))
> netif_tx_wake_queue(txq);
> }
>
> In order to get into the if-block there needs to be a 0 or negative
> return from vnet_tx_dring_avail(), yet then we're checking it again,
> with no discernible delay, for a positive value. I don't see how we
> could get to the netif_tx_wake_queue() call - shouldn't there be some
> delay, or perhaps something that might bump the underlying channel
> before checking for more available space?
This is the canonical way to deal with a race condition that exists in
pretty much every modern networking driver that does transmit without
holding onto any real locks.
Between the initial vnet_tx_dring_avail() test and the
netif_tx_stop_queue() call, a TX event can arrive and thus make TX
ring space available again.
Thus without the re-check, we could lose a wakeup.
If you scan a bunch of drivers, you'll find this construct is common,
for example tg3_start_xmit() has:
tnapi->tx_prod = entry;
if (unlikely(tg3_tx_avail(tnapi) <= (MAX_SKB_FRAGS + 1))) {
netif_tx_stop_queue(txq);
/* netif_tx_stop_queue() must be done before checking
* checking tx index in tg3_tx_avail() below, because in
* tg3_tx(), we update tx index before checking for
* netif_tx_queue_stopped().
*/
smp_mb();
if (tg3_tx_avail(tnapi) > TG3_TX_WAKEUP_THRESH(tnapi))
netif_tx_wake_queue(txq);
}
Hope this helps.
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