Re: [Xen-devel] [PATCH 2/8] x86: distinguish CPU offlining from CPU removal

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/07/18 13:06, Jan Beulich wrote:
> In order to be able to service #MC on offlined CPUs, GDT, IDT, stack,

For clarity, I'd phrase this as "CPUs, the GDT, ..." to help visually
separate CPUs as it isn't a part of the rest of the list.

> --- a/xen/arch/x86/genapic/x2apic.c
> +++ b/xen/arch/x86/genapic/x2apic.c
> @@ -201,18 +201,25 @@ static int update_clusterinfo(
>          if ( !cluster_cpus_spare )
>              cluster_cpus_spare = xzalloc(cpumask_t);
>          if ( !cluster_cpus_spare ||
> -             !alloc_cpumask_var(&per_cpu(scratch_mask, cpu)) )
> +             !cond_alloc_cpumask_var(&per_cpu(scratch_mask, cpu)) )
>              err = -ENOMEM;
>          break;
>      case CPU_UP_CANCELED:
>      case CPU_DEAD:
> +    case CPU_REMOVE:
> +        if ( park_offline_cpus == (action != CPU_REMOVE) )
> +            break;
>          if ( per_cpu(cluster_cpus, cpu) )
>          {
>              cpumask_clear_cpu(cpu, per_cpu(cluster_cpus, cpu));
>              if ( cpumask_empty(per_cpu(cluster_cpus, cpu)) )
> +            {
>                  xfree(per_cpu(cluster_cpus, cpu));
> +                per_cpu(cluster_cpus, cpu) = NULL;
> +            }
>          }
>          free_cpumask_var(per_cpu(scratch_mask, cpu));
> +        clear_cpumask_var(&per_cpu(scratch_mask, cpu));

freeing and NULL-ing the pointer at the same time is already a common
pattern  How about introducing

/* Free an allocation, and zero the pointer to it. */
#define XFREE(p)
({
    typeof(*p) *_p = (p);

    xfree(_p);
    _p = NULL;
})

and a similar wrapper for FREE_CPUMASK_VAR(p) which takes care of the
NULL-ing as well?

In time as these start to get used more widely, it should reduce the
chances of double-freeing in cleanup paths.

> @@ -63,6 +63,8 @@ static cpumask_t scratch_cpu0mask;
>  cpumask_t cpu_online_map __read_mostly;
>  EXPORT_SYMBOL(cpu_online_map);
>  
> +bool __read_mostly park_offline_cpus;
> +
>  unsigned int __read_mostly nr_sockets;
>  cpumask_t **__read_mostly socket_cpumask;
>  static cpumask_t *secondary_socket_cpumask;
> @@ -887,7 +889,7 @@ static void cleanup_cpu_root_pgt(unsigne
>      }
>  }
>  
> -static void cpu_smpboot_free(unsigned int cpu)
> +static void cpu_smpboot_free(unsigned int cpu, bool all)

Perhaps "remove" or "remove_cpu", to match the CPU_REMOVE terminology?

Also, we probably want a comment here explaining the difference between
parking and removing in terms of what infrastructure needs to remain valid.

~Andrew

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