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

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Message-ID: <CAGi-RUJ+JB972uv4DnNRNCxRyGrjP_nNNhbnZG6AyyW_ampbqw@mail.gmail.com>
Date:   Tue, 9 Jun 2020 23:03:19 +0300
From:   Ramon Fried <rfried.dev@...il.com>
To:     Tom Zanussi <zanussi@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Clark Williams <williams@...hat.com>,
        Zhang Xiao <xiao.zhang@...driver.com>
Subject: Re: [PATCH RT 1/2] tasklet: Address a race resulting in double-enqueue

On Tue, Jun 9, 2020 at 8:20 PM Tom Zanussi <zanussi@...nel.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@...utronix.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@...nel.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@...il.com>

Working, thanks a lot.