Re: [PATCH v9 02/24] x86/resctrl: kfree() rmid_ptrs from resctrl_exit()

James Morse
2024-02-20T16:02:01Z

On 20/02/2024 15:54, Thomas Gleixner wrote:
> On Tue, Feb 20 2024 at 15:46, James Morse wrote:
>> On 20/02/2024 15:27, David Hildenbrand wrote:
>>> On 13.02.24 19:44, James Morse wrote:
>>>> +static void __exit dom_data_exit(void)
>>>> +{
>>>> +    mutex_lock(&rdtgroup_mutex);
>>>> +
>>>> +    kfree(rmid_ptrs);
>>>> +    rmid_ptrs = NULL;
>>>> +
>>>> +    mutex_unlock(&rdtgroup_mutex);
>>>
>>> Just curious: is grabbing that mutex really required?
>>>
>>> Against which race are we trying to protect ourselves?
>>
>> Not a race, but its to protect against having to think about memory ordering!
>>
>>> I suspect this mutex is not required here: if we could racing with someone else, likely
>>> freeing that memory would not be safe either.
>>
>> All the accesses to that variable take the mutex, its necessary to take the mutex to
>> ensure the most recently stored values are seen. In this case the array values don't
>> matter, but rmid_ptrs is written under the mutex too.
>> There is almost certainly a control dependency that means the CPU calling dom_data_exit()
>> will see the value of rmid_ptrs from dom_data_init() - but its much simpler to check that
>> all accesses take the mutex.
>>
>> With MPAM this code can be invoked from an error IRQ signalled by the hardware, so it
>> could happen anytime.
>
> Which does not work because you can't acquire a mutex from hard
> interrupt context.

Indeed - which is why that happens via schedule_work() [0]

My point was that its non-obvious where/when this will happen, so taking the lock and
forgetting about it is the simplest thing to do.

Thanks,

James

[0]
https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/mpam_devices.c?h=mpam/snapshot/v6.7-rc2&id=7da1c7f9d9ef723f829bf44ed96e1fc4a46ef29f#n1299