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

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Date: Tue, 20 Feb 2024 16:01:57 +0000
From: James Morse <james.morse@....com>
To: Thomas Gleixner <tglx@...utronix.de>, 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 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