Re: [PATCH net v6] usbnet: fix smp_processor_id() use in preemptible context

[Alan Stern]

On Thu, Oct 08, 2026 at 09:43:38AM -0700, Jakub Kicinski wrote:
> On Thu, 8 Oct 2026 10:16:02 -0400 Alan Stern wrote:
> > > This looks odd, how did we miss this for 8 years.
> > > 
> > > Greg is probably busy, but would be good to get a confirmation
> > > from either him or some other USB expert that the callbacks
> > > can indeed be called in process context.  
> > 
> > They can be called in BH context with interrupts enabled.  Is that close 
> > enough?
> 
> BH should be fine on !RT. The patch, AFAIU, is because vhci calls
> the completion callbacks in pure, unadulterated process context.
> I'm trying to figure out how urgent the fix is, basically.
> If it's a vhci bug then the usbnet fix is at most an RT problem.
> Also if vhci is doing something wrong we don't want a truckload
> of slop patches sent our way to fix 300 drivers :S

As far as I am aware, there aren't really any guarantees on the 
context of a USB URB-completion callback.  The kerneldoc for struct urb 
in include/linux/usb.h says "The completion callback is made 
in_interrupt()", but that is most definitely out of date.

Drivers shouldn't rely on any particular context guarantees.  Not even 
whether local irqs are enabled/disabled.

Alan Stern