Subject: Re: [PATCH v6 8/9] mm: multigenerational lru: user interface

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 14 Jan 2022 10:50:05 +0530

Yu Zhao <[email-masked]> writes:

> On Thu, Jan 13, 2022 at 04:01:31PM +0530, Aneesh Kumar K.V wrote:
>> Yu Zhao <[email-masked]> writes:
>> 
>> > Add /sys/kernel/mm/lru_gen/enabled as a runtime kill switch.
>> 
>> 
>> Got the below lockdep warning while using the above kill/enable switch
>> 
>> 
>> [   84.252952] ======================================================
>> [   84.253012] WARNING: possible circular locking dependency detected
>> [   84.253074] 5.16.0-rc8-16204-g1cdcf1120b31 #511 Not tainted
>> [   84.253135] ------------------------------------------------------
>> [   84.253194] bash/2862 is trying to acquire lock:
>> [   84.253243] c0000000021ff740 (cgroup_mutex){+.+.}-{3:3}, at: store_enable+0x80/0x1510
>> [   84.253340]
>>                but task is already holding lock:
>> [   84.253410] c000000002221348 (mem_hotplug_lock){++++}-{0:0}, at: mem_hotplug_begin+0x30/0x50
>> [   84.253503]
>>                which lock already depends on the new lock.
>> 
>> [   84.255933] Chain exists of:
>>                  cgroup_mutex --> cpu_hotplug_lock --> mem_hotplug_lock
>
> Thanks. Will reverse the order between mem_hotplug_lock and
> cgroup_mutex in the next spin.

It also needs the unlocked variant of static_key_enable/disable.

[   71.204397][ T2819] bash/2819 is trying to acquire lock:                                       
[   71.204446][ T2819] c0000000020e5cd0 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_disable+0x24/0x50
[   71.204542][ T2819]                                                                                                 
[   71.204542][ T2819] but task is already holding lock:                                                               
[   71.204613][ T2819] c0000000020e5cd0 (cpu_hotplug_lock){++++}-{0:0}, at: mem_hotplug_begin+0x20/0x50
[   71.204710][ T2819]                                                                                                 
[   71.204710][ T2819] other info that might help us debug this:
[   71.204787][ T2819]  Possible unsafe locking scenario:
[   71.204787][ T2819]                    
[   71.204860][ T2819]        CPU0                                                                                                                                                                                                            
[   71.204901][ T2819]        ----
[   71.204941][ T2819]   lock(cpu_hotplug_lock);
[   71.204998][ T2819]   lock(cpu_hotplug_lock);
[   71.205053][ T2819] 
[   71.205053][ T2819]  *** DEADLOCK ***

-aneesh

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