Re: [PATCH v2 17/29] arm_mpam: Extend reset logic to allow devices to be reset any time

Fenghua Yu
2025-09-25T07:16:55Z

Hi, James,

On 9/10/25 13:42, James Morse wrote:
> cpuhp callbacks aren't the only time the MSC configuration may need to
> be reset. Resctrl has an API call to reset a class.
> If an MPAM error interrupt arrives it indicates the driver has
> misprogrammed an MSC. The safest thing to do is reset all the MSCs
> and disable MPAM.
>
> Add a helper to reset RIS via their class. Call this from mpam_disable(),
> which can be scheduled from the error interrupt handler.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Changes since v1:
> * more complete use of _srcu helpers.
> * Use guard macro for srcu.
> * Dropped a might_sleep() - something else will bark.
> ---
> drivers/resctrl/mpam_devices.c | 56 ++++++++++++++++++++++++++++++++--
> 1 file changed, 54 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index e7faf453b5d7..a9d3c4b09976 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -842,8 +842,6 @@ static int mpam_reset_ris(void *arg)
> u16 partid, partid_max;
> struct mpam_msc_ris *ris = arg;
>
> - mpam_assert_srcu_read_lock_held();
> -
> if (ris->in_reset_state)
> return 0;
>
> @@ -1340,8 +1338,56 @@ static void mpam_enable_once(void)
> mpam_partid_max + 1, mpam_pmg_max + 1);
> }
>
> +static void mpam_reset_component_locked(struct mpam_component *comp)
> +{
> + struct mpam_msc *msc;
> + struct mpam_vmsc *vmsc;
> + struct mpam_msc_ris *ris;
> +
> + lockdep_assert_cpus_held();
> +
> + guard(srcu)(&mpam_srcu);
> +

Nested locks on mpam_srcu in this call chain:

mpam_disable() -> mpam_reset_class() -> mpam_reset_class_locked() ->
mpam_component_locked()

There are redundant locks on mpam_srcu in mpam_disabled(),
mpam_reset_class_locked(), and mpam_reset_component_locked().

It's better to guard mpam_srcu only in the top function mpam_disable()
for simpler logic and lower overhead.

> list_for_each_entry_srcu(vmsc, &comp->vmsc, comp_list,
> + srcu_read_lock_held(&mpam_srcu)) {
> + msc = vmsc->msc;
> +
> + list_for_each_entry_srcu(ris, &vmsc->ris, vmsc_list,
> + srcu_read_lock_held(&mpam_srcu)) {
> + if (!ris->in_reset_state)
> + mpam_touch_msc(msc, mpam_reset_ris, ris);
> + ris->in_reset_state = true;
> + }
> + }
> +}
> +
> +static void mpam_reset_class_locked(struct mpam_class *class)
> +{
> + struct mpam_component *comp;
> +
> + lockdep_assert_cpus_held();
> +
> + guard(srcu)(&mpam_srcu);
> + list_for_each_entry_srcu(comp, &class->components, class_list,
> + srcu_read_lock_held(&mpam_srcu))
> + mpam_reset_component_locked(comp);
> +}
> +
> +static void mpam_reset_class(struct mpam_class *class)
> +{
> + cpus_read_lock();
> + mpam_reset_class_locked(class);
> + cpus_read_unlock();
> +}
> +
> +/*
> + * Called in response to an error IRQ.
> + * All of MPAMs errors indicate a software bug, restore any modified
> + * controls to their reset values.
> + */
> void mpam_disable(struct work_struct *ignored)
> {
> + int idx;
> + struct mpam_class *class;
> struct mpam_msc *msc, *tmp;
>
> mutex_lock(&mpam_cpuhp_state_lock);
> @@ -1351,6 +1397,12 @@ void mpam_disable(struct work_struct *ignored)
> }
> mutex_unlock(&mpam_cpuhp_state_lock);
>
> + idx = srcu_read_lock(&mpam_srcu);
It's better to change to guard(srcu)(&mpam_srcu);
> + list_for_each_entry_srcu(class, &mpam_classes, classes_list,
> + srcu_read_lock_held(&mpam_srcu))
> + mpam_reset_class(class);
> + srcu_read_unlock(&mpam_srcu, idx);
> +
> mutex_lock(&mpam_list_lock);
> list_for_each_entry_safe(msc, tmp, &mpam_all_msc, all_msc_list)
> mpam_msc_destroy(msc);

Thanks.

-Fenghua