Re: [PATCH RT 1/2] tasklet: Address a race resulting in double-enqueue

[Tom Zanussi <zanussi@kernel.org>]

Hi Sebastian,

On Tue, 2020-06-09 at 18:34 +0200, Sebastian Andrzej Siewior wrote:
> On 2020-06-09 11:17:53 [-0500], Tom Zanussi wrote:
> > Hi Sebastian,
> 
> Hi Tom,
> 
> > I did find a problem with the patch when configured as !SMP since
> > in
> > that case the RUN flag is never set (will send a patch for that
> > shortly), but that wouldn't be the case here.
> 
> How?
> 

My test machine with !SMP and !RT doesn't boot, and in that case we
have:

#define tasklet_trylock(t) 1
#define tasklet_tryunlock(t)    1

instead of setting/clearing the RUN flag.

So the cmpxchg with RUN+CHAIN can never work and we hit the loop.

> > #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
> > static inline int tasklet_trylock(struct tasklet_struct *t)
> > {
> >         return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state);
> > }
> 
> I can't tell from the backtrace if he runs with RT or without but I
> assumed RT. But yes, for !SMP && !RT it would explain it.
> 

Yeah, for me !SMP and RT works, but !SMP and !RT doesn't.

I had assumed he was talking about the samely configured kernel, but
apparently it's not.

Tom

> > It would help to be able to reproduce it, but I haven't been able
> > to
> > yet.
> > 
> > Tom
> > 
> 
> Sebastian