cgroup: simplify dynamic cftype addition and removal
Dynamic cftype addition and removal using cgroup_add/rm_cftypes() respectively has been quite hairy due to vfs i_mutex. As i_mutex nests outside cgroup_mutex, cgroup_mutex has to be released and regrabbed on each iteration through the hierarchy complicating the process. Now that i_mutex is no longer in play, it can be simplified. * Just holding cgroup_tree_mutex is enough. No need to meddle with cgroup_mutex. * No reason to play the unlock - relock - check serial_nr dancing. Everything can be atomically while holding cgroup_tree_mutex. * cgroup_cfts_prepare() is replaced with direct locking of cgroup_tree_mutex. * cgroup_cfts_commit() no longer fiddles with locking. It just applies the cftypes change to the existing cgroups in the hierarchy. Renamed to cgroup_cfts_apply(). Signed-off-by: NTejun Heo <tj@kernel.org> Acked-by: NLi Zefan <lizefan@huawei.com>
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