linux-kernel - Re: [RFC][PATCH 5/9] rcu: Fix rcu_idle

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Date:   Fri, 20 May 2022 14:00:08 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     frederic@...nel.org, rjw@...ysocki.net, x86@...nel.org,
        linux-kernel@...r.kernel.org, jpoimboe@...nel.org
Subject: Re: [RFC][PATCH 5/9] rcu: Fix rcu_idle_exit()

On Thu, May 19, 2022 at 11:27:55PM +0200, Peter Zijlstra wrote:
> Current rcu_idle_exit() is terminally broken because it uses
> local_irq_{save,restore}(), which are traced which uses RCU.
> 
> However, now that all the callers are sure to have IRQs disabled, we
> can remove these calls.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

This looks good to me.  If there are any callers with IRQs still
enabled, the lockdep_assert_irqs_disabled() should catch them.
And yes, after looking at the definition, I agree that it is just
fine to invoke lockdep_assert_irqs_disabled() from noinstr code.
The underlying __WARN_printf() might need an RCU_NONIDLE() or
equivalent, but if so that is a separate issue.

Acked-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/rcu/tree.c |    9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -659,7 +659,7 @@ static noinstr void rcu_eqs_enter(bool u
>   * If you add or remove a call to rcu_idle_enter(), be sure to test with
>   * CONFIG_RCU_EQS_DEBUG=y.
>   */
> -void rcu_idle_enter(void)
> +void noinstr rcu_idle_enter(void)
>  {
>  	lockdep_assert_irqs_disabled();
>  	rcu_eqs_enter(false);
> @@ -896,13 +896,10 @@ static void noinstr rcu_eqs_exit(bool us
>   * If you add or remove a call to rcu_idle_exit(), be sure to test with
>   * CONFIG_RCU_EQS_DEBUG=y.
>   */
> -void rcu_idle_exit(void)
> +void noinstr rcu_idle_exit(void)
>  {
> -	unsigned long flags;
> -
> -	local_irq_save(flags);
> +	lockdep_assert_irqs_disabled();
>  	rcu_eqs_exit(false);
> -	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_idle_exit);
>  
> 
>