# Re: \[RFC\]\[PATCH 6/7\] context\_tracking: Provide SMP ordering using RCU

- From: Petr Mladek \<pmladek@suse.com\>
- Date: 2021-09-23 12:10:08 
- List: live-patching
- Page: https://ratatoskr.run/live-patching/2021/09/1519438
- Thread: https://ratatoskr.run/live-patching/2021/09/1518181/t.md

```text
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
```