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Date:   Thu, 14 Dec 2023 11:39:13 +0000
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Subject: Re: [PATCH v7 24/24] x86/resctrl: Separate arch and fs resctrl locks
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To:     babu.moger@amd.com, x86@kernel.org, linux-kernel@vger.kernel.org
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.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
References: <20231025180345.28061-1-james.morse@arm.com>
 <20231025180345.28061-25-james.morse@arm.com>
 <9fae4348-7a1e-48ba-a5f4-c1b2ad3f0644@amd.com>
From:   James Morse <james.morse@arm.com>
In-Reply-To: <9fae4348-7a1e-48ba-a5f4-c1b2ad3f0644@amd.com>
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Hi Babu,

On 09/11/2023 20:52, Moger, Babu wrote:
> On 10/25/23 13:03, James Morse wrote:
>> resctrl has one mutex that is taken by the architecture specific code,
>> and the filesystem parts. The two interact via cpuhp, where the
>> architecture code updates the domain list. Filesystem handlers that
>> walk the domains list should not run concurrently with the cpuhp
>> callback modifying the list.
>>
>> Exposing a lock from the filesystem code means the interface is not
>> cleanly defined, and creates the possibility of cross-architecture
>> lock ordering headaches. The interaction only exists so that certain
>> filesystem paths are serialised against CPU hotplug. The CPU hotplug
>> code already has a mechanism to do this using cpus_read_lock().
>>
>> MPAM's monitors have an overflow interrupt, so it needs to be possible
>> to walk the domains list in irq context. RCU is ideal for this,
>> but some paths need to be able to sleep to allocate memory.
>>
>> Because resctrl_{on,off}line_cpu() take the rdtgroup_mutex as part
>> of a cpuhp callback, cpus_read_lock() must always be taken first.
>> rdtgroup_schemata_write() already does this.
>>
>> Most of the filesystem code's domain list walkers are currently
>> protected by the rdtgroup_mutex taken in rdtgroup_kn_lock_live().
>> The exceptions are rdt_bit_usage_show() and the mon_config helpers
>> which take the lock directly.
>>
>> Make the domain list protected by RCU. An architecture-specific
>> lock prevents concurrent writers. rdt_bit_usage_show() could
>> walk the domain list using RCU, but to keep all the filesystem
>> operations the same, this is changed to call cpus_read_lock().
>> The mon_config helpers send multiple IPIs, take the cpus_read_lock()
>> in these cases.
>>
>> The other filesystem list walkers need to be able to sleep.
>> Add cpus_read_lock() to rdtgroup_kn_lock_live() so that the
>> cpuhp callbacks can't be invoked when file system operations are
>> occurring.
>>
>> Add lockdep_assert_cpus_held() in the cases where the
>> rdtgroup_kn_lock_live() call isn't obvious.
>>
>> Resctrl's domain online/offline calls now need to take the
>> rdtgroup_mutex themselves.

> Reviewed-by: Babu Moger <babu.moger@amd.com>

Thanks!


James