cgroup: Elide write-locking threadgroup_rwsem when updating csses on an empty subtree

stable inclusion
from stable-v5.10.143
commit bfbacc2ef7b5b9261e795e361ee233e7d36efae6
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I60G94
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bfbacc2ef7b5b9261e795e361ee233e7d36efae6



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[ Upstream commit 671c11f0 ]

cgroup_update_dfl_csses() write-lock the threadgroup_rwsem as updating the
csses can trigger process migrations. However, if the subtree doesn't
contain any tasks, there aren't gonna be any cgroup migrations. This
condition can be trivially detected by testing whether
mgctx.preloaded_src_csets is empty. Elide write-locking threadgroup_rwsem if
the subtree is empty.

After this optimization, the usage pattern of creating a cgroup, enabling
the necessary controllers, and then seeding it with CLONE_INTO_CGROUP and
then removing the cgroup after it becomes empty doesn't need to write-lock
threadgroup_rwsem at all.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Cai Xinchen <caixinchen1@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Wang Weiyang <wangweiyang2@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
(cherry picked from commit 00e6868b)