cgroup: cgroup->dentry isn't a RCU pointer (febfcef6) · Commits · 戴 / test

cgroup->dentry is marked and used as a RCU pointer; however, it isn't one - the final dentry put doesn't go through call_rcu(). cgroup and dentry share the same RCU freeing rule via synchronize_rcu() in cgroup_diput() (kfree_rcu() used on cgrp is unnecessary). If cgrp is accessible under RCU read lock, so is its dentry and dereferencing cgrp->dentry doesn't need any further RCU protection or annotation. While not being accurate, before the previous patch, the RCU accessors served a purpose as memory barriers - cgroup->dentry used to be assigned after the cgroup was made visible to cgroup_path(), so the assignment and dereferencing in cgroup_path() needed the memory barrier pair. Now that list_add_tail_rcu() happens after cgroup->dentry is assigned, this no longer is necessary. Remove the now unnecessary and misleading RCU annotations from cgroup->dentry. To make up for the removal of rcu_dereference_check() in cgroup_path(), add an explicit rcu_lockdep_assert(), which asserts the dereference rule of @cgrp, not cgrp->dentry. Signed-off-by:

Tejun Heo <tj@kernel.org> Acked-by:

Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -165,7 +165,7 @@ struct cgroup {
		struct list_head files; /* my files */

		struct cgroup parent; / my parent */
		struct dentry __rcu dentry; / cgroup fs entry, RCU protected */
		struct dentry dentry; / cgroup fs entry, RCU protected */

		/* Private pointers for each registered subsystem */
		struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];

kernel/cgroup.c

+6 −5

Original line number	Diff line number	Diff line
		@@ -1756,9 +1756,11 @@ static struct kobject *cgroup_kobj;
		*/
		int cgroup_path(const struct cgroup cgrp, char buf, int buflen)
		{
		struct dentry *dentry = cgrp->dentry;
		char *start;
		struct dentry *dentry = rcu_dereference_check(cgrp->dentry,
		cgroup_lock_is_held());

		rcu_lockdep_assert(rcu_read_lock_held() \|\| cgroup_lock_is_held(),
		"cgroup_path() called without proper locking");

		if (!dentry \|\| cgrp == dummytop) {
		/*
		@@ -1782,8 +1784,7 @@ int cgroup_path(const struct cgroup cgrp, char buf, int buflen)
		if (!cgrp)
		break;

		dentry = rcu_dereference_check(cgrp->dentry,
		cgroup_lock_is_held());
		dentry = cgrp->dentry;
		if (!cgrp->parent)
		continue;
		if (--start < buf)
		@@ -4124,7 +4125,7 @@ static long cgroup_create(struct cgroup parent, struct dentry dentry,

		/* allocation complete, commit to creation */
		dentry->d_fsdata = cgrp;
		rcu_assign_pointer(cgrp->dentry, dentry);
		cgrp->dentry = dentry;
		list_add_tail(&cgrp->allcg_node, &root->allcg_list);
		list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
		root->number_of_cgroups++;

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