Re: [PATCH rcu 2/4] rcutorture: Fix rcu_torture_fwd_cb_cr() data race

[Paul E. McKenney]

On Wed, Jun 05, 2024 at 09:56:41AM +0200, Marco Elver wrote:
> On Wed, 5 Jun 2024 at 00:36, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On powerpc systems, spinlock acquisition does not order prior stores
> > against later loads.  This means that this statement:
> >
> >         rfcp->rfc_next = NULL;
> >
> > Can be reordered to follow this statement:
> >
> >         WRITE_ONCE(*rfcpp, rfcp);
> >
> > Which is then a data race with rcu_torture_fwd_prog_cr(), specifically,
> > this statement:
> >
> >         rfcpn = READ_ONCE(rfcp->rfc_next)
> >
> > KCSAN located this data race, which represents a real failure on powerpc.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Marco Elver <elver@xxxxxxxxxx>
> > Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> > Cc: <kasan-dev@xxxxxxxxxxxxxxxx>
> 
> Nice find - was this found by KCSAN's weak memory modeling, i.e. the
> report showed you that a reordered access resulted in a data race?

If I remember correctly, yes.

Even on x86, the compiler is free to reorder that WRITE_ONCE() with
unmarked accesses, so one can argue that this bug is not specific
to powerpc.

> Acked-by: Marco Elver <elver@xxxxxxxxxx>

I will apply on my next rebase, thank you!

							Thanx, Paul

> > ---
> >  kernel/rcu/rcutorture.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 44cc455e1b615..cafe047d046e8 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -2630,7 +2630,7 @@ static void rcu_torture_fwd_cb_cr(struct rcu_head *rhp)
> >         spin_lock_irqsave(&rfp->rcu_fwd_lock, flags);
> >         rfcpp = rfp->rcu_fwd_cb_tail;
> >         rfp->rcu_fwd_cb_tail = &rfcp->rfc_next;
> > -       WRITE_ONCE(*rfcpp, rfcp);
> > +       smp_store_release(rfcpp, rfcp);
> >         WRITE_ONCE(rfp->n_launders_cb, rfp->n_launders_cb + 1);
> >         i = ((jiffies - rfp->rcu_fwd_startat) / (HZ / FWD_CBS_HIST_DIV));
> >         if (i >= ARRAY_SIZE(rfp->n_launders_hist))
> > --
> > 2.40.1
> >