linux-kernel - Re: [PATCH v2 rcu/dev 1/2] rcu: Track laziness during boot and suspend

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Message-ID: <20230115162504.08ef72b0@rorschach.local.home>
Date:   Sun, 15 Jan 2023 16:25:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        fweisbec@...il.com, urezki@...il.com
Subject: Re: [PATCH v2 rcu/dev 1/2] rcu: Track laziness during boot and
 suspend

On Thu, 12 Jan 2023 00:52:22 +0000
"Joel Fernandes (Google)" <joel@...lfernandes.org> wrote:

> -- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -144,8 +144,45 @@ bool rcu_gp_is_normal(void)
>  }
>  EXPORT_SYMBOL_GPL(rcu_gp_is_normal);
>  
> -static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
> +static atomic_t rcu_async_hurry_nesting = ATOMIC_INIT(1);
> +/*
> + * Should call_rcu() callbacks be processed with urgency or are
> + * they OK being executed with arbitrary delays?
> + */
> +bool rcu_async_should_hurry(void)
> +{
> +	return !IS_ENABLED(CONFIG_RCU_LAZY) ||
> +	       atomic_read(&rcu_async_hurry_nesting);
> +}
> +EXPORT_SYMBOL_GPL(rcu_async_should_hurry);
> +
> +/**
> + * rcu_async_hurry - Make future async RCU callbacks not lazy.
> + *
> + * After a call to this function, future calls to call_rcu()
> + * will be processed in a timely fashion.
> + */
> +void rcu_async_hurry(void)
> +{
> +	if (IS_ENABLED(CONFIG_RCU_LAZY))
> +		atomic_inc(&rcu_async_hurry_nesting);
> +}
> +EXPORT_SYMBOL_GPL(rcu_async_hurry);
>  

Where do you plan on calling these externally, as they are being
marked exported?

If you allow random drivers to enable this, I can see something
enabling it and hitting an error path that causes it to never disable
it.

I wouldn't have EXPORT_SYMBOL_GPL() unless you really know that it is
needed externally. In fact, is this really needed outside of the RCU
subsystem?

-- Steve


> +/**
> + * rcu_async_relax - Make future async RCU callbacks lazy.
> + *
> + * After a call to this function, future calls to call_rcu()
> + * will be processed in a lazy fashion.
> + */
> +void rcu_async_relax(void)
> +{
> +	if (IS_ENABLED(CONFIG_RCU_LAZY))
> +		atomic_dec(&rcu_async_hurry_nesting);
> +}
> +EXPORT_SYMBOL_GPL(rcu_async_relax);
> +
> +static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);