cgroup: Fix threadgroup_rwsem <-> cpus_read_lock() deadlock

cgroup: Fix threadgroup_rwsem <-> cpus_read_lock() deadlock

Bringing up a CPU may involve creating and destroying 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 <[email protected]>
Reviewed-and-tested-by: Imran Khan <[email protected]>
Reported-and-tested-by: Xuewen Yan <[email protected]>
Fixes: 05c7b7a92cc8 ("cgroup/cpuset: Fix a race between cpuset_attach() and cpu hotplug")
Cc: [email protected] # v5.17+