linux-kernel - Re: [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT

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Message-ID: <20210817153613.l5f66czar6pajexo@linutronix.de>
Date:   Tue, 17 Aug 2021 17:36:13 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        rcu@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Steven Price <steven.price@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock()
 under PREEMPT_RT

On 2021-08-11 21:13:53 [+0100], Valentin Schneider wrote:
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 0ff5e4fb933e..11c4ff00afde 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -21,6 +21,17 @@ static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
>  	return lockdep_is_held(&rdp->nocb_lock);
>  }
>  
> +static inline int rcu_lockdep_is_held_nocb_local(struct rcu_data *rdp)
> +{
> +	return lockdep_is_held(
> +#ifdef CONFIG_PREEMPT_RT
> +		&rdp->nocb_local_lock.lock
> +#else
> +		&rdp->nocb_local_lock
> +#endif
> +	);
> +}

Now that I see it and Paul asked for it, please just use !RT version.
	return lockdep_is_held(&rdp->nocb_local_lock);

and RT will work, too.

>  static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
>  {
>  	/* Race on early boot between thread creation and assignment */
> @@ -1629,6 +1664,22 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
>  	}
>  }
>  
> +/*
> + * The invocation of rcu_core() within the RCU core kthreads remains preemptible
> + * under PREEMPT_RT, thus the offload state of a CPU could change while
> + * said kthreads are preempted. Prevent this from happening by protecting the
> + * offload state with a local_lock().
> + */
> +static void rcu_nocb_local_lock(struct rcu_data *rdp)
> +{
> +	local_lock(&rcu_data.nocb_local_lock);
> +}
> +
> +static void rcu_nocb_local_unlock(struct rcu_data *rdp)
> +{
> +	local_unlock(&rcu_data.nocb_local_lock);
> +}
> +
Do you need to pass rdp given that it is not used?

>  /* Lockdep check that ->cblist may be safely accessed. */
>  static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
>  {

Sebastian