Re: [PATCH net v2] net: usbnet: Fix the wrong netif_carrier_on() call placement

[Simon Horman <horms@kernel.org>]

+ Linus

On Mon, Aug 04, 2025 at 11:00:50AM +0100, Simon Horman wrote:
> + John Ernberg
> 
> On Sat, Aug 02, 2025 at 02:03:10AM +0700, Ammar Faizi wrote:
> > The commit in the Fixes tag breaks my laptop (found by git bisect).
> > My home RJ45 LAN cable cannot connect after that commit.
> > 
> > The call to netif_carrier_on() should be done when netif_carrier_ok()
> > is false. Not when it's true. Because calling netif_carrier_on() when
> > __LINK_STATE_NOCARRIER is not set actually does nothing.
> > 
> > Cc: Armando Budianto <sprite@gnuweeb.org>
> > Cc: stable@vger.kernel.org
> > Closes: https://lore.kernel.org/netdev/0752dee6-43d6-4e1f-81d2-4248142cccd2@gnuweeb.org
> > Fixes: 0d9cfc9b8cb1 ("net: usbnet: Avoid potential RCU stall on LINK_CHANGE event")
> > Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
> > ---
> > 
> > v2:
> >   - Rebase on top of the latest netdev/net tree. The previous patch was
> >     based on 0d9cfc9b8cb1. Line numbers have changed since then.
> > 
> >  drivers/net/usb/usbnet.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)

It seems this has escalated a bit as it broke things for Linus while
he was travelling. He tested this patch and it resolved the problem.
Which I think counts for something.

https://lore.kernel.org/netdev/CAHk-=wgkvNuGCDUMMs9bW9Mz5o=LcMhcDK_b2ThO6_T7cquoEQ@mail.gmail.com/

I have looked over the patch and it appears to me that it addresses a
straightforward logic error: a check was added to turn the carrier on only
if it is already on. Which seems a bit nonsensical. And presumably the
intention was to add the check for the opposite case.

This patch addresses that problem.

So let me try and nudge this on a bit by providing a tag.

Reviewed-by: Simon Horman <horms@kernel.org>

> > 
> > diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> > index a38ffbf4b3f0..a1827684b92c 100644
> > --- a/drivers/net/usb/usbnet.c
> > +++ b/drivers/net/usb/usbnet.c
> > @@ -1114,31 +1114,31 @@ static const struct ethtool_ops usbnet_ethtool_ops = {
> >  };
> >  
> >  /*-------------------------------------------------------------------------*/
> >  
> >  static void __handle_link_change(struct usbnet *dev)
> >  {
> >  	if (!test_bit(EVENT_DEV_OPEN, &dev->flags))
> >  		return;
> >  
> >  	if (!netif_carrier_ok(dev->net)) {
> > +		if (test_and_clear_bit(EVENT_LINK_CARRIER_ON, &dev->flags))
> > +			netif_carrier_on(dev->net);
> > +
> >  		/* kill URBs for reading packets to save bus bandwidth */
> >  		unlink_urbs(dev, &dev->rxq);
> >  
> >  		/*
> >  		 * tx_timeout will unlink URBs for sending packets and
> >  		 * tx queue is stopped by netcore after link becomes off
> >  		 */
> >  	} else {
> > -		if (test_and_clear_bit(EVENT_LINK_CARRIER_ON, &dev->flags))
> > -			netif_carrier_on(dev->net);
> > -
> >  		/* submitting URBs for reading packets */
> >  		queue_work(system_bh_wq, &dev->bh_work);
> >  	}
> >  
> >  	/* hard_mtu or rx_urb_size may change during link change */
> >  	usbnet_update_max_qlen(dev);
> >  
> >  	clear_bit(EVENT_LINK_CHANGE, &dev->flags);
> >  }
> >  
> > -- 
> > Ammar Faizi
> > 
> > 
>