Re: [PATCH v2 2/2] rcutorture: Update ->extendables check for lazy preemption

[Boqun Feng <boqun.feng@gmail.com>]

[Cc Joel]

On Sun, Feb 23, 2025 at 08:43:10PM -0800, Boqun Feng wrote:
> The rcutorture_one_extend_check() function's second last check assumes
> that "preempt_count() & PREEMPT_MASK" is non-zero only if
> RCUTORTURE_RDR_PREEMPT or RCUTORTURE_RDR_SCHED bit is set.
> 
> This works for preemptible RCU and for non-preemptible RCU running in
> a non-preemptible kernel.  But it fails for non-preemptible RCU running
> in a preemptible kernel because then rcu_read_lock() is just
> preempt_disable(), which increases preempt count.
> 
> This commit therefore adjusts this check to take into account the case
> fo non-preemptible RCU running in a preemptible kernel.
> 
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
>  kernel/rcu/rcutorture.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 280bff706017..4cae119dece8 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1901,8 +1901,16 @@ static void rcutorture_one_extend_check(char *s, int curstate, int new, int old,
>  	WARN_ONCE(cur_ops->extendables &&
>  		  !(curstate & (RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH)) &&
>  		  (preempt_count() & SOFTIRQ_MASK), ROEC_ARGS);
> -	WARN_ONCE(cur_ops->extendables &&
> -		  !(curstate & (RCUTORTURE_RDR_PREEMPT | RCUTORTURE_RDR_SCHED)) &&
> +
> +	/*
> +	 * non-preemptible RCU in a preemptible kernel uses preempt_disable()
> +	 * as rcu_read_lock().
> +	 */
> +	mask = RCUTORTURE_RDR_PREEMPT | RCUTORTURE_RDR_SCHED;
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
> +		mask |= RCUTORTURE_RDR_RCU_1 | RCUTORTURE_RDR_RCU_2;
> +
> +	WARN_ONCE(cur_ops->extendables && !(curstate & mask) &&
>  		  (preempt_count() & PREEMPT_MASK), ROEC_ARGS);
>  
>  	/*
> -- 
> 2.39.5 (Apple Git-154)
> 
>