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

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Date: Tue, 20 Feb 2024 16:54:10 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: James Morse <james.morse@....com>, David Hildenbrand <david@...hat.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Cc: Fenghua Yu <fenghua.yu@...el.com>, Reinette Chatre
 <reinette.chatre@...el.com>, Ingo Molnar <mingo@...hat.com>, Borislav
 Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.com>, Babu Moger
 <Babu.Moger@....com>, shameerali.kolothum.thodi@...wei.com, 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
Subject: Re: [PATCH v9 02/24] x86/resctrl: kfree() rmid_ptrs from
 resctrl_exit()

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