linux-kernel - Re: [PATCH] rcutorture: Fix operator precedence bug in exp

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Message-ID: <4bbae00e-f078-4f1e-a1e0-b1cdc72981de@paulmck-laptop>
Date: Mon, 22 Dec 2025 17:12:18 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Josh Triplett <josh@...htriplett.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>, rcu@...r.kernel.org
Subject: Re: [PATCH] rcutorture: Fix operator precedence bug in exp_current()
 invocation

On Mon, Dec 22, 2025 at 07:44:07PM -0500, Joel Fernandes wrote:
> Fix incorrect operator precedence in the conditional that determines
> when to call exp_current() during torture testing. The modulo
> by 0xff appears to do nothing.
> 
> Fix by adding parentheses:
> 
>     !(torture_random(&rand) % 0xff)
> 
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>

Good eyes, thank you!

Unfortunately, Chris Mason beat you to it:

50d8a5d2a94c ("rcutorture: Correctly compute probability to invoke ->exp_current()")

I will add your Reported-by to that commit on my next rebase.

							Thanx, Paul

> ---
>  kernel/rcu/rcutorture.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 82588ceed9da..9873a1d54c46 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1749,7 +1749,7 @@ rcu_torture_writer(void *arg)
>  					ulo[i] = cur_ops->get_comp_state();
>  				gp_snap = cur_ops->start_gp_poll();
>  				rcu_torture_writer_state = RTWS_POLL_WAIT;
> -				if (cur_ops->exp_current && !torture_random(&rand) % 0xff)
> +				if (cur_ops->exp_current && !(torture_random(&rand) % 0xff))
>  					cur_ops->exp_current();
>  				while (!cur_ops->poll_gp_state(gp_snap)) {
>  					gp_snap1 = cur_ops->get_gp_state();
> @@ -1771,7 +1771,7 @@ rcu_torture_writer(void *arg)
>  					cur_ops->get_comp_state_full(&rgo[i]);
>  				cur_ops->start_gp_poll_full(&gp_snap_full);
>  				rcu_torture_writer_state = RTWS_POLL_WAIT_FULL;
> -				if (cur_ops->exp_current && !torture_random(&rand) % 0xff)
> +				if (cur_ops->exp_current && !(torture_random(&rand) % 0xff))
>  					cur_ops->exp_current();
>  				while (!cur_ops->poll_gp_state_full(&gp_snap_full)) {
>  					cur_ops->get_gp_state_full(&gp_snap1_full);
> -- 
> 2.34.1
>