Re: [PATCH net] xsk: correct tx_ring_empty_descs count statistics

[Stanislav Fomichev]

On 03/31, Stanislav Fomichev wrote:
> On 03/29, Wang Liang wrote:
> > The tx_ring_empty_descs count may be incorrect, when set the XDP_TX_RING
> > option but do not reserve tx ring. Because xsk_poll() try to wakeup the
> > driver by calling xsk_generic_xmit() for non-zero-copy mode. So the
> > tx_ring_empty_descs count increases once the xsk_poll()is called:
> > 
> >   xsk_poll
> >     xsk_generic_xmit
> >       __xsk_generic_xmit
> >         xskq_cons_peek_desc
> >           xskq_cons_read_desc
> >             q->queue_empty_descs++;
> > 
> > To avoid this count error, add check for tx descs before send msg in poll.
> > 
> > Fixes: df551058f7a3 ("xsk: Fix crash in poll when device does not support ndo_xsk_wakeup")
> > Signed-off-by: Wang Liang <wangliang74@xxxxxxxxxx>
> 
> Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>

Hmm, wait, I stumbled upon xskq_has_descs again and it looks only at
cached prod/cons. How is it supposed to work when the actual tx
descriptor is posted? Is there anything besides xskq_cons_peek_desc from
__xsk_generic_xmit that refreshes cached_prod?