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

[Marco Elver]

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?

Acked-by: Marco Elver <elver@xxxxxxxxxx>

> ---
>  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
>