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

Zheng, Lv
2015-04-27T03:16:07Z

Hi,

> From: Borislav Petkov [mailto:bp@alien8.de]
> Sent: Friday, March 27, 2015 5:23 PM
>
> From: Jiri Kosina <jkosina@suse.cz>
>
> 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@suse.cz>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> ---
> 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