Re: [Xen-devel] [PATCH 4/7] xen/arm: When CPU dies, free percpu area immediatelly

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 11/04/18 14:19, Mirela Simonovic wrote:
> Freeing percpu area is done when a non-boot CPU is disabled upon suspend.
> This use to be scheduled for execution after a period of time, what caused
> the following racing issues. If CPU is enabled after it is disabled and
> before the freeing of percpu area is performed, Xen would crash upon
> initializing percpu area because per cpu offset is not marked as
> INVALID_PERCPU_AREA (this suppose to happen when cpu area is freed).
> To resolve the racing issue, free percpu area right away instead
> scheduling it for later.
The reason of using the RCU is you want to make sure that none of the 
other CPUs will access that percpu data before freeing it. So I don't 
think this patch is valid.
It looks like to me a rcu barrier is missing after calling cpu_down 
somewhere in the CPU off path. I am not entirely sure where.
Cheers,

> Signed-off-by: Mirela Simonovic <mirela.simonovic@xxxxxxxxxx>
> ---
>   xen/arch/arm/percpu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/arch/arm/percpu.c b/xen/arch/arm/percpu.c
> index 25442c48fe..e4e8405f43 100644
> --- a/xen/arch/arm/percpu.c
> +++ b/xen/arch/arm/percpu.c
> @@ -46,7 +46,7 @@ static void free_percpu_area(unsigned int cpu)
>   {
>       struct free_info *info = &per_cpu(free_info, cpu);
>       info->cpu = cpu;
> -    call_rcu(&info->rcu, _free_percpu_area);
> +    _free_percpu_area(&info->rcu);
>   }
>   
>   static int cpu_percpu_callback(
--
Julien Grall

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