Re: [Xen-devel] [PATCH 2/6] xen: add helper for calling notifier_call_chain() to common/cpu.c

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/03/2019 13:11, Juergen Gross wrote:
> Add a helper cpu_notifier_call_chain() to call notifier_call_chain()
> for a cpu with a specified action, returning an errno value.
>
> This avoids coding the same pattern multiple times.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  xen/common/cpu.c | 50 +++++++++++++++++++++-----------------------------
>  1 file changed, 21 insertions(+), 29 deletions(-)
>
> diff --git a/xen/common/cpu.c b/xen/common/cpu.c
> index 836c62f97f..c436c0de7f 100644
> --- a/xen/common/cpu.c
> +++ b/xen/common/cpu.c
> @@ -71,11 +71,18 @@ void __init register_cpu_notifier(struct notifier_block 
> *nb)
>      spin_unlock(&cpu_add_remove_lock);
>  }
>  
> +static int cpu_notifier_call_chain(unsigned int cpu, unsigned long action,
> +                                   struct notifier_block **nb)
> +{
> +    void *hcpu = (void *)(long)cpu;
> +    int notifier_rc = notifier_call_chain(&cpu_chain, action, hcpu, nb);
> +
> +    return (notifier_rc == NOTIFY_DONE) ? 0 : notifier_to_errno(notifier_rc);
> +}
> +
>  static void _take_cpu_down(void *unused)
>  {
> -    void *hcpu = (void *)(long)smp_processor_id();
> -    int notifier_rc = notifier_call_chain(&cpu_chain, CPU_DYING, hcpu, NULL);
> -    BUG_ON(notifier_rc != NOTIFY_DONE);
> +    BUG_ON(cpu_notifier_call_chain(smp_processor_id(), CPU_DYING, NULL));

Where possible, we're trying to remove side effects from macros.

Could I please talk you into writing this as:

int rc = cpu_notifier_call_chain(smp_processor_id(), CPU_DYING, NULL);

BUG_ON(rc);

An alternative might be to have a:

static void cpu_notifier_call_chain_nofail(...)

wrapper as this seems to be a common pattern.  (Ideally longterm, it
might be better to pass the nofail intention into the notifier chain
itself so we can identify which callback had a problem, but thats
definitely not something for here.)

~Andrew

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