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

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Message-ID: <1249c061-dde7-4966-9c8c-2fa958fad37b@nvidia.com>
Date: Thu, 25 Sep 2025 00:16:48 -0700
From: Fenghua Yu <fenghuay@...dia.com>
To: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org
Cc: D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 baisheng.gao@...soc.com, Jonathan Cameron <jonathan.cameron@...wei.com>,
 Rob Herring <robh@...nel.org>, Rohit Mathew <rohit.mathew@....com>,
 Rafael Wysocki <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>, Hanjun Guo
 <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 17/29] arm_mpam: Extend reset logic to allow devices to
 be reset any time

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@....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