cgroup: break kernfs active_ref protection in cgroup directory operations (e1b2dc17) · Commits · 戴 / test

cgroup_tree_mutex should nest above the kernfs active_ref protection; however, cgroup_create() and cgroup_rename() were grabbing cgroup_tree_mutex while under kernfs active_ref protection. This has actualy possibility to lead to deadlocks in case these operations race against cgroup_rmdir() which invokes kernfs_remove() on directory kernfs_node while holding cgroup_tree_mutex. Neither cgroup_create() or cgroup_rename() requires active_ref protection. The former already has enough synchronization through cgroup_lock_live_group() and the latter doesn't care, so this can be fixed by updating both functions to break all active_ref protections before grabbing cgroup_tree_mutex. While this patch fixes the immediate issue, it probably needs further work in the long term - kernfs directories should enable lockdep annotations and maybe the better way to handle this is marking directory nodes as not needing active_ref protection rather than breaking it in each operation. Signed-off-by:

Tejun Heo <tj@kernel.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/cgroup.c

+26 −1

Original line number	Diff line number	Diff line
		@@ -2324,6 +2324,14 @@ static int cgroup_rename(struct kernfs_node kn, struct kernfs_node new_parent,
		if (cgroup_sane_behavior(cgrp))
		return -EPERM;

		/*
		* We're gonna grab cgroup_tree_mutex which nests outside kernfs
		* active_ref. kernfs_rename() doesn't require active_ref
		* protection. Break them before grabbing cgroup_tree_mutex.
		*/
		kernfs_break_active_protection(new_parent);
		kernfs_break_active_protection(kn);

		mutex_lock(&cgroup_tree_mutex);
		mutex_lock(&cgroup_mutex);

		@@ -2331,6 +2339,9 @@ static int cgroup_rename(struct kernfs_node kn, struct kernfs_node new_parent,

		mutex_unlock(&cgroup_mutex);
		mutex_unlock(&cgroup_tree_mutex);

		kernfs_unbreak_active_protection(kn);
		kernfs_unbreak_active_protection(new_parent);
		return ret;
		}

		@@ -3778,8 +3789,22 @@ static int cgroup_mkdir(struct kernfs_node parent_kn, const char name,
		umode_t mode)
		{
		struct cgroup *parent = parent_kn->priv;
		int ret;

		return cgroup_create(parent, name, mode);
		/*
		* cgroup_create() grabs cgroup_tree_mutex which nests outside
		* kernfs active_ref and cgroup_create() already synchronizes
		* properly against removal through cgroup_lock_live_group().
		* Break it before calling cgroup_create().
		*/
		cgroup_get(parent);
		kernfs_break_active_protection(parent_kn);

		ret = cgroup_create(parent, name, mode);

		kernfs_unbreak_active_protection(parent_kn);
		cgroup_put(parent);
		return ret;
		}

		/*

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