linux-kernel - Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU

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Date:   Thu, 23 Sep 2021 14:10:01 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     gor@...ux.ibm.com, jpoimboe@...hat.com, jikos@...nel.org,
        mbenes@...e.cz, mingo@...nel.org, linux-kernel@...r.kernel.org,
        joe.lawrence@...hat.com, fweisbec@...il.com, tglx@...utronix.de,
        hca@...ux.ibm.com, svens@...ux.ibm.com, sumanthk@...ux.ibm.com,
        live-patching@...r.kernel.org, paulmck@...nel.org
Subject: Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU

On Wed 2021-09-22 13:05:12, Peter Zijlstra wrote:
> Use rcu_user_{enter,exit}() calls to provide SMP ordering on context
> tracking state stores:
> 
> __context_tracking_exit()
>   __this_cpu_write(context_tracking.state, CONTEXT_KERNEL)
>   rcu_user_exit()
>     rcu_eqs_exit()
>       rcu_dynticks_eqs_eit()
>         rcu_dynticks_inc()
>           atomic_add_return() /* smp_mb */
> 
> __context_tracking_enter()
>   rcu_user_enter()
>     rcu_eqs_enter()
>       rcu_dynticks_eqs_enter()
>         rcu_dynticks_inc()
> 	  atomic_add_return() /* smp_mb */
>   __this_cpu_write(context_tracking.state, state)
> 
> This separates USER/KERNEL state with an smp_mb() on each side,
> therefore, a user of context_tracking_state_cpu() can say the CPU must
> pass through an smp_mb() before changing.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/context_tracking_state.h |   12 ++++++++++++
>  kernel/context_tracking.c              |    7 ++++---
>  2 files changed, 16 insertions(+), 3 deletions(-)
> 
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -45,11 +45,23 @@ static __always_inline bool context_trac
>  {
>  	return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
>  }
> +
> +static __always_inline bool context_tracking_state_cpu(int cpu)
> +{
> +	struct context_tracking *ct = per_cpu_ptr(&context_tracking);

Missing cpu parameter:

	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);

> +
> +	if (!context_tracking_enabled() || !ct->active)
> +		return CONTEXT_DISABLED;
> +
> +	return ct->state;
> +}

Best Regards,
Petr