Re: [RFC PATCH net-next 1/2] net: napi: Fix interrupts permanently disabled during busy poll

[Martin Karsten]

On 2026-04-28 19:40, Jakub Kicinski wrote:
> On Tue, 28 Apr 2026 17:51:30 +0000 Dragos Tatulea wrote:
> > Under certain conditions a queue can be left out with interrupts
> > disabled and with the napi re-scheduling timer permanently stopped.
> > This behaviour is triggered by the napi busy poll path when
> > gro-flush-timeout and defer-hard-irq are set. Here's a sequence of
> > operations:
> >
> > 1. Busy poll starts, NAPI_STATE_SCHED is set to avoid rescheduling napi
> > from the timer.
> >
> > 2. During napi poll, driver disables interrupts due to being in poll
> > mode (napi_complete_done() returns false because napi->state has
> > NAPIF_STATE_IN_BUSY_POLL set).
>
> Why does the driver have IRQs disabled in busy poll?

The problems occurs in irq deferral mode when both gro-flush-timeout and 
defer-hard-irqs are nonzero and NIC interrupts are disabled.

> > 3. At the end of the busy poll (busy_poll_stop()):
> >    3.1 napi timer is scheduled and skip_schedule is set (due to config)
> >    3.2 napi->poll() is called:
> >      - driver poll() processes exactly budget packets
> >        and exits early => napi not scheduled.
> >        (interrupts are still disabled at this point)
> >    3.3 Since napi poll processed budget packets, __busy_poll_stop()
> >      is called with skip_schedule set => napi is not scheduled here
> >      either.
>
> with skip_schedule it calls:
>
> 	clear_bit(NAPI_STATE_SCHED, &napi->state);
>
> > 4. If the napi timer from 3.1 gets to be triggered due to slow napi poll
> > or some other reason, the timer will run with no effect (due to
> > NAPI_STATE_SCHED being set).
>
> And here you claim STATE_SCHED is still set?

Labelling this with number 4. might be misleading, sorry! The concern is 
that a short enough timer (compared to the duration of the driver poll) 
can be triggered before the NAPI_STATE_SCHED bit is cleared at the end 
of Step 3.3.

Thanks,
Martin