Re: [RESEND PATCH 4/4] can: c_can: cache frames to operate as a true FIFO

[Marc Kleine-Budde]

On 05.08.2021 22:16:06, Dario Binacchi wrote:
> > > --- a/drivers/net/can/c_can/c_can.h
> > > +++ b/drivers/net/can/c_can/c_can.h
> > > @@ -200,6 +200,7 @@ struct c_can_priv {
> > >  	atomic_t sie_pending;
> > >  	unsigned long tx_dir;
> > >  	int last_status;
> > > +	spinlock_t tx_lock;
> > 
> > What does the spin lock protect?
[...]
> > > @@ -483,7 +469,11 @@ static netdev_tx_t c_can_start_xmit(struct sk_buff *skb,
> > >  	if (c_can_get_tx_free(tx_ring) == 0)
> > >  		netif_stop_queue(dev);
> > >  
> > > -	obj = idx + priv->msg_obj_tx_first;
> > > +	spin_lock_bh(&priv->tx_lock);
> > 
> > What does the spin_lock protect? The ndo_start_xmit function is properly
> > serialized by the networking core.
> > 
> 
> The spin_lock protects the access to the IF_TX interface.

How? You only use the spin_lock in c_can_start_xmit(), but not anywhere
else.

> Enabling the transmission of cached messages occur inside interrupt

The call chain is c_can_poll() -> c_can_do_tx(), and c_can_poll() is
called from NAPI, which is not the IRQ handler.

> and the use of the IF_RX interface, which would avoid the use of the
> spinlock, has not been validated by the tests.

What do you mean be has not been validated?

The driver already uses IF_RX to avoid concurrent access in
c_can_do_tx() for c_can_inval_tx_object() [1], why not use IF_RX for
c_can_object_put(), too?

[1] https://lore.kernel.org/r/20210302215435.18286-4-dariobin@xxxxxxxxx

Marc

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