From ae04dd5ef1801784eb0717de2141e094d7544195 Mon Sep 17 00:00:00 2001 From: Shakeel Butt Date: Sun, 24 Oct 2021 23:19:14 -0700 Subject: [PATCH] cgroup: reduce dependency on cgroup_mutex [ Upstream commit be288169712f3dea0bc6b50c00b3ab53d85f1435 ] Currently cgroup_get_from_path() and cgroup_get_from_id() grab cgroup_mutex before traversing the default hierarchy to find the kernfs_node corresponding to the path/id and then extract the linked cgroup. Since cgroup_mutex is still held, it is guaranteed that the cgroup will be alive and the reference can be taken on it. However similar guarantee can be provided without depending on the cgroup_mutex and potentially reducing avenues of cgroup_mutex contentions. The kernfs_node's priv pointer is RCU protected pointer and with just rcu read lock we can grab the reference on the cgroup without cgroup_mutex. So, remove cgroup_mutex from them. Signed-off-by: Shakeel Butt Signed-off-by: Tejun Heo Stable-dep-of: df02452f3df0 ("cgroup: cgroup_get_from_id() must check the looked-up kn is a directory") Signed-off-by: Sasha Levin --- kernel/cgroup/cgroup.c | 51 ++++++++++++++++++++++++++++---------------------- 1 file changed, 29 insertions(+), 22 deletions(-) diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 75c3881..97282d6 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -6021,17 +6021,20 @@ struct cgroup *cgroup_get_from_id(u64 id) struct kernfs_node *kn; struct cgroup *cgrp = NULL; - mutex_lock(&cgroup_mutex); kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id); if (!kn) - goto out_unlock; + goto out; + + rcu_read_lock(); - cgrp = kn->priv; - if (cgroup_is_dead(cgrp) || !cgroup_tryget(cgrp)) + cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv); + if (cgrp && !cgroup_tryget(cgrp)) cgrp = NULL; + + rcu_read_unlock(); + kernfs_put(kn); -out_unlock: - mutex_unlock(&cgroup_mutex); +out: return cgrp; } EXPORT_SYMBOL_GPL(cgroup_get_from_id); @@ -6585,30 +6588,34 @@ struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss) * * Find the cgroup at @path on the default hierarchy, increment its * reference count and return it. Returns pointer to the found cgroup on - * success, ERR_PTR(-ENOENT) if @path doesn't exist and ERR_PTR(-ENOTDIR) - * if @path points to a non-directory. + * success, ERR_PTR(-ENOENT) if @path doesn't exist or if the cgroup has already + * been released and ERR_PTR(-ENOTDIR) if @path points to a non-directory. */ struct cgroup *cgroup_get_from_path(const char *path) { struct kernfs_node *kn; - struct cgroup *cgrp; - - mutex_lock(&cgroup_mutex); + struct cgroup *cgrp = ERR_PTR(-ENOENT); kn = kernfs_walk_and_get(cgrp_dfl_root.cgrp.kn, path); - if (kn) { - if (kernfs_type(kn) == KERNFS_DIR) { - cgrp = kn->priv; - cgroup_get_live(cgrp); - } else { - cgrp = ERR_PTR(-ENOTDIR); - } - kernfs_put(kn); - } else { - cgrp = ERR_PTR(-ENOENT); + if (!kn) + goto out; + + if (kernfs_type(kn) != KERNFS_DIR) { + cgrp = ERR_PTR(-ENOTDIR); + goto out_kernfs; } - mutex_unlock(&cgroup_mutex); + rcu_read_lock(); + + cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv); + if (!cgrp || !cgroup_tryget(cgrp)) + cgrp = ERR_PTR(-ENOENT); + + rcu_read_unlock(); + +out_kernfs: + kernfs_put(kn); +out: return cgrp; } EXPORT_SYMBOL_GPL(cgroup_get_from_path); -- 2.7.4