Re: [kvm-unit-tests PATCH v2 2/3] lib/on-cpus: Add barrier after func call

[Andrew Jones <andrew.jones@linux.dev>]

On Thu, Oct 31, 2024 at 01:39:51PM +0100, Andrew Jones wrote:
> It's reasonable for users of on_cpu() and on_cpumask() to assume
> they can read data that 'func' has written when the call completes.
> Ensure the caller doesn't observe a completion (target cpus are again
> idle) without also being able to observe any writes which were made
> by func(). Do so by adding barriers to implement the following
> 
>  target-cpu                                   calling-cpu
>  ----------                                   -----------
>  func() /* store something */                 /* wait for target to be idle */
>  smp_wmb();                                   smp_rmb();
>  set_cpu_idle(smp_processor_id(), true);      /* load what func() stored */
> 
> Signed-off-by: Andrew Jones <andrew.jones@linux.dev>

I added a

Suggested-by: Alexandru Elisei <alexandru.elisei@arm.com>

to this patch.

> ---
>  lib/on-cpus.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/lib/on-cpus.c b/lib/on-cpus.c
> index f6072117fa1b..356f284be61b 100644
> --- a/lib/on-cpus.c
> +++ b/lib/on-cpus.c
> @@ -79,6 +79,7 @@ void do_idle(void)
>  			smp_wait_for_event();
>  		smp_rmb();
>  		on_cpu_info[cpu].func(on_cpu_info[cpu].data);
> +		smp_wmb(); /* pairs with the smp_rmb() in on_cpu() and on_cpumask() */
>  	}
>  }
>  
> @@ -145,12 +146,14 @@ void on_cpumask(const cpumask_t *mask, void (*func)(void *data), void *data)
>  		smp_wait_for_event();
>  	for_each_cpu(cpu, mask)
>  		cpumask_clear_cpu(me, &on_cpu_info[cpu].waiters);
> +	smp_rmb(); /* pairs with the smp_wmb() in do_idle() */
>  }
>  
>  void on_cpu(int cpu, void (*func)(void *data), void *data)
>  {
>  	on_cpu_async(cpu, func, data);
>  	cpu_wait(cpu);
> +	smp_rmb(); /* pairs with the smp_wmb() in do_idle() */
>  }
>  
>  void on_cpus(void (*func)(void *data), void *data)
> -- 
> 2.47.0
> 
> 
> -- 
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