Re: [PATCH cgroup/for-6.0-fixes] cgroup: Fix threadgroup_rwsem <-> cpus_read_lock() deadlock

[Imran Khan]

Hello Tejun,

On 16/8/22 9:27 am, Tejun Heo wrote:
> Bringing up a CPU may involve creating new tasks which requires read-locking
> threadgroup_rwsem, so threadgroup_rwsem nests inside cpus_read_lock().
> However, cpuset's ->attach(), which may be called with thredagroup_rwsem
> write-locked, also wants to disable CPU hotplug and acquires
> cpus_read_lock(), leading to a deadlock.
> 
> Fix it by guaranteeing that ->attach() is always called with CPU hotplug
> disabled and removing cpus_read_lock() call from cpuset_attach().
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
> Hello, sorry about the delay.
> 
> So, the previous patch + the revert isn't quite correct because we sometimes
> elide both cpus_read_lock() and threadgroup_rwsem together and
> cpuset_attach() woudl end up running without CPU hotplug enabled. Can you
> please test whether this patch fixes the problem?
> 

This fixes the issue seen in my setup. As my setup is 5.4 based I used
cgroup_attach_lock/unlock(true) in the backport version of your patch.

Feel free to add my

Reviewed-and-tested-by: Imran Khan <imran.f.khan@xxxxxxxxxx>

Thanks,
-- Imran