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

[Thomas Gleixner]

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.

Thanks,

        tglx