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

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Message-ID: <88f45c7f-9769-478b-9abd-749ec8f8d43b@arm.com>
Date: Fri, 26 Sep 2025 18:52:49 +0100
From: James Morse <james.morse@....com>
To: Markus Elfring <Markus.Elfring@....de>, linux-acpi@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Cc: LKML <linux-kernel@...r.kernel.org>,
 Amit Singh Tomar <amitsinght@...vell.com>,
 Baisheng Gao <baisheng.gao@...soc.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Ben Horgan
 <ben.horgan@....com>, Carl Worth <carl@...amperecomputing.com>,
 Catalin Marinas <catalin.marinas@....com>,
 D Scott Phillips <scott@...amperecomputing.com>,
 Danilo Krummrich <dakr@...nel.org>, Dave Martin <Dave.Martin@....com>,
 David Hildenbrand <david@...hat.com>, Drew Fustini <dfustini@...libre.com>,
 Fenghua Yu <fenghuay@...dia.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Hanjun Guo <guohanjun@...wei.com>, Jamie Iles <quic_jiles@...cinc.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>, Koba Ko <kobak@...dia.com>,
 Len Brown <lenb@...nel.org>, Linu Cherian <lcherian@...vell.com>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Peter Newman <peternewman@...gle.com>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Rob Herring <robh@...nel.org>,
 Rohit Mathew <rohit.mathew@....com>,
 Shanker Donthineni <sdonthineni@...dia.com>,
 Sudeep Holla <sudeep.holla@....com>, Shaopeng Tan
 <tan.shaopeng@...itsu.com>, Wang ShaoBo <bobo.shaobowang@...wei.com>,
 Will Deacon <will@...nel.org>, Xin Hao <xhao@...ux.alibaba.com>
Subject: Re: [PATCH v2 08/29] arm_mpam: Add the class and component structures
 for firmware described ris

Hi Markus,

On 11/09/2025 17:30, 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.

Thanks,

James