Re: [PATCH v2 08/29] arm_mpam: Add the class and component structures for firmware described ris

[James Morse <james.morse@arm.com>]

Hi Markus,

On 26/09/2025 19:15, Markus Elfring wrote:
>>> …
>>>> +++ b/drivers/resctrl/mpam_devices.c
>>> …
>>>>> +int mpam_ris_create(struct mpam_msc *msc, u8 ris_idx,
>>>> +		    enum mpam_class_types type, u8 class_id, int component_id)
>>>> +{
>>>> +	int err;
>>>> +
>>>> +	mutex_lock(&mpam_list_lock);
>>>> +	err = mpam_ris_create_locked(msc, ris_idx, type, class_id,
>>>> +				     component_id);
>>>> +	mutex_unlock(&mpam_list_lock);
>>> …
>>>
>>> Under which circumstances would you become interested to apply a statement
>>> like “guard(mutex)(&mpam_list_lock);”?
>>> https://elixir.bootlin.com/linux/v6.17-rc5/source/include/linux/mutex.h#L228
>>
>> None! The bit of this you cut out is a call to mpam_free_garbage() which calls
>> synchronize_srcu(). That may sleep for a while. The whole point of the deferred free-ing
>> is it does not happen under the lock. The 'guard' magic means the compiler gets to choose
>> when to call unlock.
> 
> How does this feedback fit to the proposed addition of a mutex_lock()/mutex_unlock()
> call combination (which might be achievable also with another programming interface)?

Right - I've muddled the horde of "must use guard srcu" with the horde of "must use guard
mutex". In this case I'd still prefer we don't spuriously hold the write side lock when
doing the deferred free-ing.


Thanks,

James