On Wed, 2024-05-15 at 13:12 +0200, Marc Kleine-Budde wrote:
> On 14.05.2024 15:34:01, Vitor Soares wrote:
> > > > +void mcp251xfd_tx_obj_write_sync(struct work_struct *ws)
> > > > +{
> > > > + struct mcp251xfd_priv *priv = container_of(ws, struct
> > > > mcp251xfd_priv,
> > > > + tx_work);
> > > > + struct mcp251xfd_tx_obj *tx_obj = priv->tx_work_obj;
> > > > + struct mcp251xfd_tx_ring *tx_ring = priv->tx;
> > > > + int err;
> > > > +
> > > > + err = spi_sync(priv->spi, &tx_obj->msg);
> > > > + if (err)
> > > > + mcp251xfd_tx_failure_drop(priv, tx_ring, err);
> > > > +
> > > > + priv->tx_work_obj = NULL;
> > >
> > > Race condition:
> > > - after spi_sync() the CAN frame is send
> > > - after the TX complete IRQ the TX queue is restarted
> > > - the xmit handler might get BUSY
> > > - fill the tx_work_obj again
>
> You can avoid the race condition by moving "priv->tx_work_obj = NULL;"
> in front of the "spi_sync();". Right?
>
> > > > +}
> > > > +
> > > > static int mcp251xfd_tx_obj_write(const struct mcp251xfd_priv *priv,
> > > > struct mcp251xfd_tx_obj *tx_obj)
> > > > {
> > > > @@ -175,7 +210,7 @@ netdev_tx_t mcp251xfd_start_xmit(struct sk_buff
> > > > *skb,
> > > > if (can_dev_dropped_skb(ndev, skb))
> > > > return NETDEV_TX_OK;
> > > >
> > > > - if (mcp251xfd_tx_busy(priv, tx_ring))
> > > > + if (mcp251xfd_tx_busy(priv, tx_ring) || priv->tx_work_obj)
> > >
> > > This should not happen, but better save than sorry.
> >
> > As there is the race condition you mentioned above, on this condition:
> > priv->tx_work_obj = tx_obj --> xmit will return NETDEV_TX_BUSY
> >
> > or
> >
> > priv->tx_work_obj = NULL --> It goes through the rest of the code or
> > the workqueue may sleep after setting tx_work_obj to NULL. Should I
> > use work_busy() here instead or do you have another suggestion?
>
> Yes, introduce mcp251xfd_work_busy().
>
I'll implement it.
> I'm not sure what happens if the xmit is called between the
> "priv->tx_work_obj = NULL" and the end of the work. Will queue_work()
> return false, as the queue is still running?
>From the test I did so far, my understanding is the following:
If mcp251xfd_tx_obj_write doesn't fail, everything is OK.
if mcp251xfd_tx_obj_write fails with EBUSY
- stop netif queue
- fill the tx_work_obj
- start worker
queue_work() doesn't return false even when work_busy() = true.
- xmit handler return, and wait netif_wake_queue()
- the work handler waits until the previous job gets done before starting the
next one.
- after the TX completes IRQ, the TX queue is restarted
If the TX queue is restarted immediately after queue_work(), tx_work_obj is
filled, making the xmit handler return NETDEV_TX_BUSY.
The tests were done with a delay after priv->tx_work_obj = NULL.
Best regards,
Vitor Soares
