Re: [PATCH] net: usbnet: Avoid potential RCU stall on LINK_CHANGE event

[John Ernberg <john.ernberg@actia.se>]

Hi Jakub,

On Fri, Jul 18, 2025 at 04:18:25PM -0700, Jakub Kicinski wrote:
> On Fri, 18 Jul 2025 09:07:26 +0000 John Ernberg wrote:
> > > Thanks for the analysis, I think I may have misread the code.
> > > What I was saying is that we are restoring the carrier while
> > > we are still processing the previous carrier off event in
> > > the workqueue. My thinking was that if we deferred the
> > > netif_carrier_on() to the workqueue this race couldn't happen.
> > > 
> > > usbnet_bh() already checks netif_carrier_ok() - we're kinda duplicating
> > > the carrier state with this RX_PAUSED workaround.
> > > 
> > > I don't feel strongly about this, but deferring the carrier_on()
> > > the the workqueue would be a cleaner solution IMO.
> > >   
> > 
> > I've been thinking about this idea, but I'm concerned for the opposite 
> > direction. I cannot think of a way to fully guarantee that the carrier 
> > isn't turned on again incorrectly if an off gets queued.
> > 
> > The most I came up with was adding an extra flag bit to set carrier on, 
> > and then test_and_clear_bit() it in the __handle_link_change() function.
> > And also clear_bit() in the usbnet_link_change() function if an off 
> > arrives. I cannot convince myself that there isn't a way for that to go 
> > sideways. But perhaps that would be robust enough?
> 
> I think it should be robust enough.. Unless my grep skills are failing
> me - no drivers which call usbnet_link_change() twiddle the link state
> directly.
> 
> Give it a go, if you think your initial patch is cleaner -- it's fine.
> 

Apologies for the delay, I was stuck in a higher priority issue.

I've tested this approach and it looks promising. Will send this approach
as a v2 later today.

Thank you for the guidance, very much appreciated.

Best regards // John Ernberg