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

[Tom Zanussi <zanussi@kernel.org>]

On Tue, 2020-06-09 at 23:03 +0300, Ramon Fried wrote:
> On Tue, Jun 9, 2020 at 8:20 PM Tom Zanussi <zanussi@kernel.org>
> wrote:
> > 
> > Hi Ramon,
> > 
> > On Tue, 2020-06-09 at 20:14 +0300, Ramon Fried wrote:
> > > 
> > > On June 9, 2020 8:10:43 PM GMT+03:00, Sebastian Andrzej Siewior <
> > > bigeasy@linutronix.de> wrote:
> > > > On 2020-06-09 20:07:06 [+0300], Ramon Fried wrote:
> > > > > Indeed
> > > > >  I'm truly sorry, I thought our crash kernel is configured as
> > > > > RT
> > > > > as
> > > > 
> > > > well.
> > > > > so, as I understand, if I build the RT kernel without preempt
> > > > > enabled
> > > > 
> > > > I can hit this bug?
> > > > 
> > > > Don't worry, I should have been better with the details in the
> > > > log.
> > > > 
> > > > So you should _always_ hit the warning/bug if you compile a
> > > > kernel
> > > > without SMP and RT. If you enable one of these then everything
> > > > should
> > > > be
> > > > fine.
> > > 
> > > Would there be a fix for that?
> > 
> > I haven't tested the fix yet, but can you try the below patch and
> > see
> > if it fixes your broken case?
> > 
> > [PATCH] tasklet: Fix UP case for tasklet CHAINED state
> > 
> > commit 62d0a2a30cd0 (tasklet: Address a race resulting in
> > double-enqueue) addresses a problem that can result in a tasklet
> > being
> > enqueued on two cpus at the same time by combining the RUN flag
> > with a
> > new CHAINED flag, and relies on the combination to be present in
> > order
> > to zero it out, which can never happen on (!SMP and
> > !PREEMPT_RT_FULL)
> > because the RUN flag is SMP/PREEMPT_RT_FULL-only.
> > 
> > So make sure the above commit is only applied for the SMP ||
> > PREEMPT_RT_FULL case.
> > 
> > Signed-off-by: Tom Zanussi <zanussi@kernel.org>
> > ---
> >  kernel/softirq.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > index 73dae64bfc9c..9bad7a16dc61 100644
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> > @@ -947,10 +947,12 @@ static void __tasklet_schedule_common(struct
> > tasklet_struct *t,
> >          * is locked before adding it to the list.
> >          */
> >         if (test_bit(TASKLET_STATE_SCHED, &t->state)) {
> > +#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
> >                 if (test_and_set_bit(TASKLET_STATE_CHAINED, &t-
> > >state))
> > {
> >                         tasklet_unlock(t);
> >                         return;
> >                 }
> > +#endif
> >                 t->next = NULL;
> >                 *head->tail = t;
> >                 head->tail = &(t->next);
> > @@ -1044,7 +1046,11 @@ static void tasklet_action_common(struct
> > softirq_action *a,
> >  again:
> >                 t->func(t->data);
> > 
> > +#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
> >                 while (cmpxchg(&t->state, TASKLET_STATEF_RC, 0) !=
> > TASKLET_STATEF_RC) {
> > +#else
> > +               while (!tasklet_tryunlock(t)) {
> > +#endif
> >                         /*
> >                          * If it got disabled meanwhile, bail out:
> >                          */
> > --
> > 2.17.1
> > 
> > 
> 
> Tested-By: Ramon Fried <rfried.dev@gmail.com>
> 
> Working, thanks a lot.

OK, great, thanks for testing and reporting this.

Tom