linux-kernel - Re: [PATCH RFC v2 rcu 2/8] srcu: Create an srcu_read_lock_nmisafe() and srcu_read_unlock

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Date:   Sun, 2 Oct 2022 17:55:16 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH RFC v2 rcu 2/8] srcu: Create an srcu_read_lock_nmisafe()
 and srcu_read_unlock_nmisafe()

On Thu, Sep 29, 2022 at 11:07:25AM -0700, Paul E. McKenney wrote:
> @@ -1090,7 +1121,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
>  	int ss_state;
>  
>  	check_init_srcu_struct(ssp);
> -	idx = srcu_read_lock(ssp);
> +	idx = __srcu_read_lock_nmisafe(ssp);

Why do we need to force the atomic based version here (even if CONFIG_NEED_SRCU_NMI_SAFE=y)?

>  	ss_state = smp_load_acquire(&ssp->srcu_size_state);
>  	if (ss_state < SRCU_SIZE_WAIT_CALL)
>  		sdp = per_cpu_ptr(ssp->sda, 0);
> @@ -1123,7 +1154,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
>  		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
>  	else if (needexp)
>  		srcu_funnel_exp_start(ssp, sdp_mynode, s);
> -	srcu_read_unlock(ssp, idx);
> +	__srcu_read_unlock_nmisafe(ssp, idx);
>  	return s;
>  }
>  
> @@ -1427,13 +1458,13 @@ void srcu_barrier(struct srcu_struct *ssp)
>  	/* Initial count prevents reaching zero until all CBs are posted. */
>  	atomic_set(&ssp->srcu_barrier_cpu_cnt, 1);
>  
> -	idx = srcu_read_lock(ssp);
> +	idx = __srcu_read_lock_nmisafe(ssp);

And same here?

Thanks.

>  	if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
>  		srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, 0));
>  	else
>  		for_each_possible_cpu(cpu)
>  			srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, cpu));
> -	srcu_read_unlock(ssp, idx);
> +	__srcu_read_unlock_nmisafe(ssp, idx);
>  
>  	/* Remove the initial count, at which point reaching zero can happen. */
>  	if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
> -- 
> 2.31.1.189.g2e36527f23
>