Re: [Xen-devel] [PATCH v2] x86/cpu: Sync any remaining RCU callbacks after CPU up/down

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.02.2020 18:25, Igor Druzhinin wrote:
> --- a/xen/arch/x86/sysctl.c
> +++ b/xen/arch/x86/sysctl.c
> @@ -78,8 +78,11 @@ static void l3_cache_get(void *arg)
>  long cpu_up_helper(void *data)
>  {
>      unsigned int cpu = (unsigned long)data;
> -    int ret = cpu_up(cpu);
> +    int ret;
>  
> +    /* Flush potentially scheduled RCU work from preceding CPU offline */
> +    rcu_barrier();
> +    ret = cpu_up(cpu);
>      if ( ret == -EBUSY )
>      {
>          /* On EBUSY, flush RCU work and have one more go. */
> @@ -104,7 +107,11 @@ long cpu_up_helper(void *data)
>  long cpu_down_helper(void *data)
>  {
>      int cpu = (unsigned long)data;
> -    int ret = cpu_down(cpu);
> +    int ret;
> +
> +    /* Flush potentially scheduled RCU work from preceding CPU online */
> +    rcu_barrier();
> +    ret = cpu_down(cpu);
>      if ( ret == -EBUSY )
>      {
>          /* On EBUSY, flush RCU work and have one more go. */
> 

There are more calls to cpu_up() / cpu_down(), most notably in
core_parking.c. Wouldn't it be better to make the barrier part
of the two functions? This would the also cover non-x86 in
case an arch wants to support offlining/onlining of CPUs.

Jan

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