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

Petr Mladek
2021-09-23T12:10:05Z

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@infradead.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