On Fri, Oct 14, 2022 at 10:45:04PM +0000, Joel Fernandes wrote:
> On Thu, Oct 13, 2022 at 07:22:42PM +0200, Frederic Weisbecker wrote:
> > Using the NMI-unsafe reader API from within NMIs is very likely to be
> > buggy for three reasons:
> >
> > 1) NMIs aren't strictly re-entrant (a pending nested NMI will execute
> > at the end of the current one) so it should be fine to use a
> > non-atomic increment here. However breakpoints can still interrupt
> > NMIs and if a breakpoint callback has a reader on that same ssp, a
> > racy increment can happen.
> >
> > 2) If the only reader site for a given ssp is in an NMI, RCU is definetly
> definitely
> > a better choice over SRCU.
>
> Just checking - because NMI are by definition not-preemptibe, so SRCU over
> RCU doesn't make much sense right?
Agreed. But you never know...
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
I will apply on the next rebase (after today's rebase), thank you!
Thanx, Paul
> thanks,
>
> - Joel
>
> >
> > 3) Because of the previous reason (2), an ssp having an SRCU read side
> > critical section in an NMI is likely to have another one from a task
> > context.
> >
> > For all these reasons, warn if an nmi unsafe reader API is used from an
> > NMI.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > ---
> > kernel/rcu/srcutree.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index c54142374793..8b7ef1031d89 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -642,6 +642,8 @@ static void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe)
> >
> > if (!IS_ENABLED(CONFIG_PROVE_RCU))
> > return;
> > + /* NMI-unsafe use in NMI is a bad sign */
> > + WARN_ON_ONCE(!nmi_safe && in_nmi());
> > sdp = raw_cpu_ptr(ssp->sda);
> > old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety);
> > if (!old_nmi_safe_mask) {
> > --
> > 2.25.1
> >
