Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

Frederic Weisbecker
2021-09-22T11:38:24Z

On Wed, Sep 22, 2021 at 01:27:31PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-09-22 13:10:12 [+0200], Frederic Weisbecker wrote:
> > On Wed, Sep 22, 2021 at 08:32:08AM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2021-09-22 01:45:18 [+0200], Frederic Weisbecker wrote:
> > > >
> > > > Also while at it, I'm asking again: traditionally softirqs could assume that
> > > > manipulating a local state was safe against !irq_count() code fiddling with
> > > > the same state on the same CPU.
> > > >
> > > > Now with preemptible softirqs, that assumption can be broken anytime. RCU was
> > > > fortunate enough to have a warning for that. But who knows how many issues like
> > > > this are lurking?
> > >
> > > If "local state" is modified then it is safe as long as it is modified
> > > within a local_bh_disable() section. And we are in this section while
> > > invoking a forced-threaded interrupt. The special part about RCU is
> > > that it is used in_irq() as part of core-code.
> >
> > But local_bh_disable() was deemed for protecting from interrupting softirqs,
> > not the other way around (softirqs being preempted by other tasks). The latter
> > semantic is new and nobody had that in mind until softirqs have been made
> > preemptible.
> >
> > For example:
> >
> > CPU 0
> > -----------------------------------------------
> > SOFTIRQ RANDOM TASK
> > ------ -----------
> > int *X = &per_cpu(CPUX, 0) int *X = &per_cpu(CPUX, 0)
> > int A, B; WRITE_ONCE(*X, 0);
> > WRITE_ONCE(*X, 1);
> > A = READ_ONCE(*X);
> > B = READ_ONCE(*X);
> >
> >
> > We used to have the guarantee that A == B. That's not true anymore. Now
> > some new explicit local_bh_disable() should be carefully placed on RANDOM_TASK
> > where it wasn't necessary before. RCU is not that special in this regard.
>
> The part with rcutree.use_softirq=0 on RT does not make it any better,
> right?

The rcuc kthread disables softirqs before calling rcu_core(), so it behaves
pretty much the same as a softirq. Or am I missing something?

> So you rely on some implicit behaviour which breaks with RT such as:
>
> CPU 0
> -----------------------------------------------
> RANDOM TASK-A RANDOM TASK-B
> ------ -----------
> int *X = &per_cpu(CPUX, 0) int *X = &per_cpu(CPUX, 0)
> int A, B;
> spin_lock(&D);
> spin_lock(&C);
> WRITE_ONCE(*X, 0);
> A = READ_ONCE(*X);
> WRITE_ONCE(*X, 1);
> B = READ_ONCE(*X);
>
> while spinlock C and D are just random locks not related to CPUX but it
> just happens that they are held at that time. So for !RT you guarantee
> that A == B while it is not the case on RT.

Not sure which spinlocks you are referring to here. Also most RCU spinlocks
are raw.

>
> Sebastian