Re: Query regarding deadlock involving cgroup_threadgroup_rwsem and cpu_hotplug_lock

[Mukesh Ojha <quic_mojha@quicinc.com>]

Looks like these patches are the fixes.

https://lore.kernel.org/all/YtDvN0wJ6CKaEPN8@slm.duckdns.org/#r

Would let Tejun confirm this .

-Mukesh

On 7/20/2022 4:36 PM, Mukesh Ojha wrote:
> Hi,
> 
> On 7/20/2022 8:57 AM, Imran Khan wrote:
>> Hello everyone,
>>
>> I am seeing a deadlock between cgroup_threadgroup_rwsem and 
>> cpu_hotplug_lock in
>> 5.4 kernel.
>>
>> Due to some missing drivers I don't have this test setup for latest 
>> upstream
>> kernel but looking at the code the issue seems to be present in the 
>> latest
>> kernel as well. If needed I can provide stack traces and other 
>> relevant info
>> from the vmcore that I have got from 5.4 setup.
>>
>> The description of the problem is as follows (I am using 5.19-rc7 as 
>> reference
>> below):
>>
>> __cgroup_procs_write acquires cgroup_threadgroup_rwsem via
>> cgroup_procs_write_start and then invokes cgroup_attach_task. Now
>> cgroup_attach_task can invoke following call chain:
>>
>> cgroup_attach_task --> cgroup_migrate --> cgroup_migrate_execute --> 
>> cpuset_attach
>>
>> Here cpuset_attach tries to take cpu_hotplug_lock.
>>
>> But by this time if some other context
>>
>> 1. is already in the middle of cpu hotplug and has acquired 
>> cpu_hotplug_lock in
>> _cpu_up but
>> 2. has not yet reached CPUHP_ONLINE state and
>> 3. one of the intermediate hotplug states (in my case 
>> CPUHP_AP_ONLINE_DYN ) has
>> a callback which involves creation of a thread (or invocation of 
>> copy_process
>> via some other path) the invoked copy_process will get blocked on
>> cgroup_threadgroup_rwsem in following call chain:
>>
>>     copy_process --> cgroup_can_fork --> cgroup_css_set_fork -->
>> cgroup_threadgroup_change_begin
> 
> Similar discussion is at [1], not sure on the conclusion.
> 
> [1]
> https://lore.kernel.org/lkml/20220705123705.764-1-xuewen.yan@unisoc.com/
> 
> -Mukesh
> 
>>
>>
>> I am looking for suggestions to fix this deadlock.
>>
>> Or if I am missing something in the above analysis and the above mention
>> scenario can't happen in latest upstream kernel, then please let me 
>> know as that
>> would help me in back porting relevant changes to 5.4 kernel because 
>> the issue
>> definitely exists in 5.4 kernel.
>>
>> Thanks,
>> -- Imran