Re: [PATCH net] net: fix hangup on napi_disable for threaded napi

[Paolo Abeni <pabeni@redhat.com>]

On Wed, 2021-03-31 at 18:41 -0700, Jakub Kicinski wrote:
> On Thu,  1 Apr 2021 00:46:18 +0200 Paolo Abeni wrote:
> > I hit an hangup on napi_disable(), when the threaded
> > mode is enabled and the napi is under heavy traffic.
> > 
> > If the relevant napi has been scheduled and the napi_disable()
> > kicks in before the next napi_threaded_wait() completes - so
> > that the latter quits due to the napi_disable_pending() condition,
> > the existing code leaves the NAPI_STATE_SCHED bit set and the
> > napi_disable() loop waiting for such bit will hang.
> > 
> > Address the issue explicitly clearing the SCHED_BIT on napi_thread
> > termination, if the thread is owns the napi.
> > 
> > Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support")
> > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > ---
> >  net/core/dev.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index b4c67a5be606d..e2e716ba027b8 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -7059,6 +7059,14 @@ static int napi_thread_wait(struct napi_struct *napi)
> >  		set_current_state(TASK_INTERRUPTIBLE);
> >  	}
> >  	__set_current_state(TASK_RUNNING);
> > +
> > +	/* if the thread owns this napi, and the napi itself has been disabled
> > +	 * in-between napi_schedule() and the above napi_disable_pending()
> > +	 * check, we need to clear the SCHED bit here, or napi_disable
> > +	 * will hang waiting for such bit being cleared
> > +	 */
> > +	if (test_bit(NAPI_STATE_SCHED_THREADED, &napi->state) || woken)
> > +		clear_bit(NAPI_STATE_SCHED, &napi->state);
> 
> Not sure this covers 100% of the cases. We depend on the ability to go
> through schedule() "unnecessarily" when the napi gets scheduled after
> we go into TASK_INTERRUPTIBLE.

Empirically this patch fixes my test case (napi_disable/napi_enable in
a loop with the relevant napi under a lot of UDP traffic).

If I understand correctly, the critical scenario you see is something
alike:

CPU0			CPU1					CPU2
			// napi_threaded_poll() main loop
			napi_complete_done()
			// napi_threaded_poll() loop completes
	
napi_schedule()
// set SCHED bit
// NOT set SCHED_THREAD
// wake_up_process() is
// a no op
								napi_disable()
								// set DISABLE bit
			
			napi_thread_wait()
			set_current_state(TASK_INTERRUPTIBLE);
			// napi_thread_wait() loop completes,
			// SCHED_THREAD bit is cleared and
			// wake is false
	
> If we just check woken outside of the loop it may be false even though
> we got a "wake event".

I think in the above example even the normal processing will be
fooled?!? e.g. even without the napi_disable(), napi_thread_wait() will
 will miss the event/will not understand to it really own the napi and
will call schedule().

It looks a different problem to me ?!?

I *think* that replacing inside the napi_thread_wait() loop:

	if (test_bit(NAPI_STATE_SCHED_THREADED, &napi->state) || woken) 

with:

	unsigned long state = READ_ONCE(napi->state);

	if (state & NAPIF_STATE_SCHED &&
	    !(state & (NAPIF_STATE_IN_BUSY_POLL | NAPIF_STATE_DISABLE)) 

should solve it and should also allow removing the
NAPI_STATE_SCHED_THREADED bit. I feel like I'm missing some relevant
point here.

> Looking closer now I don't really understand where we ended up with
> disable handling :S  Seems like the thread exits on napi_disable(),
> but is reaped by netif_napi_del(). Some drivers (*cough* nfp) will
> go napi_disable() -> napi_enable()... and that will break. 
> 
> Am I missing something?
> 
> Should we not stay in the wait loop on napi_disable()?

Here I do not follow?!? Modulo the tiny race (which i was unable to
trigger so far) above napi_disable()/napi_enable() loops work correctly
here.

Could you please re-phrase?

Thanks!

Paolo