RE: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader

[Zheng, Lv]

Hi,

> From: Borislav Petkov [mailto:bp@xxxxxxxxx]
> Sent: Friday, March 27, 2015 5:23 PM
> 
> From: Jiri Kosina <jkosina@xxxxxxx>
> 
> Since GHES sources are global, we theoretically need only a single CPU
> reading them per NMI instead of a thundering herd of CPUs waiting on a
> spinlock in NMI context for no reason at all.
> 
> Do that.
> 
> Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> ---
>  drivers/acpi/apei/ghes.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
> index 94a44bad5576..2bfd53cbfe80 100644
> --- a/drivers/acpi/apei/ghes.c
> +++ b/drivers/acpi/apei/ghes.c
> @@ -729,10 +729,10 @@ static struct llist_head ghes_estatus_llist;
>  static struct irq_work ghes_proc_irq_work;
> 
>  /*
> - * NMI may be triggered on any CPU, so ghes_nmi_lock is used for
> - * mutual exclusion.
> + * NMI may be triggered on any CPU, so ghes_in_nmi is used for
> + * having only one concurrent reader.
>   */
> -static DEFINE_RAW_SPINLOCK(ghes_nmi_lock);
> +static atomic_t ghes_in_nmi = ATOMIC_INIT(0);
> 
>  static LIST_HEAD(ghes_nmi);
> 
> @@ -840,7 +840,9 @@ static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
>  	struct ghes *ghes;
>  	int sev, ret = NMI_DONE;
> 
> -	raw_spin_lock(&ghes_nmi_lock);
> +	if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
> +		return ret;
> +

Just a simple question.
Why not just using cmpxchg here instead of atomic_add_unless so that no atomic_dec will be needed.

Thanks and best regards
-Lv

>  	list_for_each_entry_rcu(ghes, &ghes_nmi, list) {
>  		if (ghes_read_estatus(ghes, 1)) {
>  			ghes_clear_estatus(ghes);
> @@ -863,7 +865,7 @@ static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
>  #ifdef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
>  	irq_work_queue(&ghes_proc_irq_work);
>  #endif
> -	raw_spin_unlock(&ghes_nmi_lock);
> +	atomic_dec(&ghes_in_nmi);
>  	return ret;
>  }
> 
> --
> 2.3.3

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