Re: Query regarding deadlock involving cgroup_threadgroup_rwsem and cpu_hotplug_lock

[Mukesh Ojha <quic_mojha@quicinc.com>]

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