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From: Thomas Gleixner <tglx@linutronix.de>
To: James Morse <james.morse@arm.com>, David Hildenbrand <david@redhat.com>,
 x86@kernel.org, linux-kernel@vger.kernel.org
Cc: Fenghua Yu <fenghua.yu@intel.com>, Reinette Chatre
 <reinette.chatre@intel.com>, Ingo Molnar <mingo@redhat.com>, Borislav
 Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>, Babu Moger
 <Babu.Moger@amd.com>, shameerali.kolothum.thodi@huawei.com, D Scott
 Phillips OS <scott@os.amperecomputing.com>, carl@os.amperecomputing.com,
 lcherian@marvell.com, bobo.shaobowang@huawei.com,
 tan.shaopeng@fujitsu.com, baolin.wang@linux.alibaba.com, Jamie Iles
 <quic_jiles@quicinc.com>, Xin Hao <xhao@linux.alibaba.com>,
 peternewman@google.com, dfustini@baylibre.com, amitsinght@marvell.com
Subject: Re: [PATCH v9 02/24] x86/resctrl: kfree() rmid_ptrs from
 resctrl_exit()
In-Reply-To: <59ebda21-6164-4dff-9ba8-956d5a715048@arm.com>
References: <20240213184438.16675-1-james.morse@arm.com>
 <20240213184438.16675-3-james.morse@arm.com>
 <52f81c45-efa7-42c7-86f4-fc1084b1d57a@redhat.com>
 <59ebda21-6164-4dff-9ba8-956d5a715048@arm.com>
Date: Tue, 20 Feb 2024 16:54:10 +0100
Message-ID: <87v86jgmhp.ffs@tglx>
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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)
>>> +{
>>> +=C2=A0=C2=A0=C2=A0 mutex_lock(&rdtgroup_mutex);
>>> +
>>> +=C2=A0=C2=A0=C2=A0 kfree(rmid_ptrs);
>>> +=C2=A0=C2=A0=C2=A0 rmid_ptrs =3D NULL;
>>> +
>>> +=C2=A0=C2=A0=C2=A0 mutex_unlock(&rdtgroup_mutex);
>>=20
>> Just curious: is grabbing that mutex really required?
>>=20
>> Against which race are we trying to protect ourselves?
>
> Not a race, but its to protect against having to think about memory order=
ing!
>
>> I suspect this mutex is not required here: if we could racing with someo=
ne else, likely
>> freeing that memory would not be safe either.
>
> All the accesses to that variable take the mutex, its necessary to take t=
he mutex to
> ensure the most recently stored values are seen. In this case the array v=
alues 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 simpl=
er to check that
> all accesses take the mutex.
>
> With MPAM this code can be invoked from an error IRQ signalled by the har=
dware, so it
> could happen anytime.

Which does not work because you can't acquire a mutex from hard
interrupt context.

Thanks,

        tglx