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cgroup: Reorganize css_set_lock and kernfs path processing
The commit 74e4b95 incorrectly wrapped kernfs_walk_and_get (might_sleep) under css_set_lock (spinlock). css_set_lock is needed by __cset_cgroup_from_root to ensure stable cset->cgrp_links but not for kernfs_walk_and_get. We only need to make sure that the returned root_cgrp won't be freed under us. This is given in the case of global root because it is static (cgrp_dfl_root.cgrp). When the root_cgrp is lower in the hierarchy, it is pinned by cgroup_ns->root_cset (and `current` task cannot switch namespace asynchronously so ns_proxy pins cgroup_ns). Note this reasoning won't hold for root cgroups in v1 hierarchies, therefore create a special-cased helper function just for the default hierarchy. Fixes: 74e4b95 ("cgroup: Honor caller's cgroup NS when resolving path") Reported-by: Dan Carpenter <[email protected]> Signed-off-by: Michal Koutný <[email protected]> Signed-off-by: Tejun Heo <[email protected]>
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