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Commit 87fb54f1 authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup: css iterations and css_from_dir() are safe under cgroup_mutex 



Currently, all css iterations and css_from_dir() require RCU read lock
whether the caller is holding cgroup_mutex or not, which is
unnecessarily restrictive.  They are all safe to use under
cgroup_mutex without holding RCU read lock.

Factor out cgroup_assert_mutex_or_rcu_locked() from css_from_id() and
apply it to all css iteration functions and css_from_dir().

v2: cgroup_assert_mutex_or_rcu_locked() definition doesn't need to be
    inside CONFIG_PROVE_RCU ifdef as rcu_lockdep_assert() is always
    defined and conditionalized.  Move it outside of the ifdef block.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent e58e1ca4

kernel/cgroup.c

+30 −26
Original line number Diff line number Diff line
@@ -93,6 +93,11 @@ static DEFINE_MUTEX(cgroup_mutex); 


static DEFINE_MUTEX(cgroup_root_mutex);



#define cgroup_assert_mutex_or_rcu_locked()				\

	rcu_lockdep_assert(rcu_read_lock_held() ||			\

			   lockdep_is_held(&cgroup_mutex),		\

			   "cgroup_mutex or RCU read lock required");



/*

 * cgroup destruction makes heavy use of work items and there can be a lot

 * of concurrent destructions.  Use a separate workqueue so that cgroup
@@ -2897,9 +2902,9 @@ static void cgroup_enable_task_cg_lists(void) 
 * @parent_css: css whose children to walk

 *

 * This function returns the next child of @parent_css and should be called

 * under RCU read lock.  The only requirement is that @parent_css and

 * @pos_css are accessible.  The next sibling is guaranteed to be returned

 * regardless of their states.

 * under either cgroup_mutex or RCU read lock.  The only requirement is

 * that @parent_css and @pos_css are accessible.  The next sibling is

 * guaranteed to be returned regardless of their states.

 */

struct cgroup_subsys_state *

css_next_child(struct cgroup_subsys_state *pos_css,
@@ -2909,7 +2914,7 @@ css_next_child(struct cgroup_subsys_state *pos_css, 
	struct cgroup *cgrp = parent_css->cgroup;

	struct cgroup *next;



	WARN_ON_ONCE(!rcu_read_lock_held());

	cgroup_assert_mutex_or_rcu_locked();



	/*

	 * @pos could already have been removed.  Once a cgroup is removed,
@@ -2956,10 +2961,10 @@ EXPORT_SYMBOL_GPL(css_next_child); 
 * to visit for pre-order traversal of @root's descendants.  @root is

 * included in the iteration and the first node to be visited.

 *

 * While this function requires RCU read locking, it doesn't require the

 * whole traversal to be contained in a single RCU critical section.  This

 * function will return the correct next descendant as long as both @pos

 * and @root are accessible and @pos is a descendant of @root.

 * While this function requires cgroup_mutex or RCU read locking, it

 * doesn't require the whole traversal to be contained in a single critical

 * section.  This function will return the correct next descendant as long

 * as both @pos and @root are accessible and @pos is a descendant of @root.

 */

struct cgroup_subsys_state *

css_next_descendant_pre(struct cgroup_subsys_state *pos,
@@ -2967,7 +2972,7 @@ css_next_descendant_pre(struct cgroup_subsys_state *pos, 
{

	struct cgroup_subsys_state *next;



	WARN_ON_ONCE(!rcu_read_lock_held());

	cgroup_assert_mutex_or_rcu_locked();



	/* if first iteration, visit @root */

	if (!pos)
@@ -2998,17 +3003,17 @@ EXPORT_SYMBOL_GPL(css_next_descendant_pre); 
 * is returned.  This can be used during pre-order traversal to skip

 * subtree of @pos.

 *

 * While this function requires RCU read locking, it doesn't require the

 * whole traversal to be contained in a single RCU critical section.  This

 * function will return the correct rightmost descendant as long as @pos is

 * accessible.

 * While this function requires cgroup_mutex or RCU read locking, it

 * doesn't require the whole traversal to be contained in a single critical

 * section.  This function will return the correct rightmost descendant as

 * long as @pos is accessible.

 */

struct cgroup_subsys_state *

css_rightmost_descendant(struct cgroup_subsys_state *pos)

{

	struct cgroup_subsys_state *last, *tmp;



	WARN_ON_ONCE(!rcu_read_lock_held());

	cgroup_assert_mutex_or_rcu_locked();



	do {

		last = pos;
@@ -3044,10 +3049,11 @@ css_leftmost_descendant(struct cgroup_subsys_state *pos) 
 * to visit for post-order traversal of @root's descendants.  @root is

 * included in the iteration and the last node to be visited.

 *

 * While this function requires RCU read locking, it doesn't require the

 * whole traversal to be contained in a single RCU critical section.  This

 * function will return the correct next descendant as long as both @pos

 * and @cgroup are accessible and @pos is a descendant of @cgroup.

 * While this function requires cgroup_mutex or RCU read locking, it

 * doesn't require the whole traversal to be contained in a single critical

 * section.  This function will return the correct next descendant as long

 * as both @pos and @cgroup are accessible and @pos is a descendant of

 * @cgroup.

 */

struct cgroup_subsys_state *

css_next_descendant_post(struct cgroup_subsys_state *pos,
@@ -3055,7 +3061,7 @@ css_next_descendant_post(struct cgroup_subsys_state *pos, 
{

	struct cgroup_subsys_state *next;



	WARN_ON_ONCE(!rcu_read_lock_held());

	cgroup_assert_mutex_or_rcu_locked();



	/* if first iteration, visit leftmost descendant which may be @root */

	if (!pos)
@@ -5217,16 +5223,16 @@ __setup("cgroup_disable=", cgroup_disable); 
 * @dentry: directory dentry of interest

 * @ss: subsystem of interest

 *

 * Must be called under RCU read lock.  The caller is responsible for

 * pinning the returned css if it needs to be accessed outside the RCU

 * critical section.

 * Must be called under cgroup_mutex or RCU read lock.  The caller is

 * responsible for pinning the returned css if it needs to be accessed

 * outside the critical section.

 */

struct cgroup_subsys_state *css_from_dir(struct dentry *dentry,

					 struct cgroup_subsys *ss)

{

	struct cgroup *cgrp;



	WARN_ON_ONCE(!rcu_read_lock_held());

	cgroup_assert_mutex_or_rcu_locked();



	/* is @dentry a cgroup dir? */

	if (!dentry->d_inode ||
@@ -5249,9 +5255,7 @@ struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss) 
{

	struct cgroup *cgrp;



	rcu_lockdep_assert(rcu_read_lock_held() ||

			   lockdep_is_held(&cgroup_mutex),

			   "css_from_id() needs proper protection");

	cgroup_assert_mutex_or_rcu_locked();



	cgrp = idr_find(&ss->root->cgroup_idr, id);

	if (cgrp)