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cgroup: remove cgroup_tree_mutex
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cgroup_tree_mutex was introduced to work around the circular
dependency between cgroup_mutex and kernfs active protection - some
kernfs file and directory operations needed cgroup_mutex putting
cgroup_mutex under active protection but cgroup also needs to be able
to access cgroup hierarchies and cftypes to determine which
kernfs_nodes need to be removed.  cgroup_tree_mutex nested above both
cgroup_mutex and kernfs active protection and used to protect the
hierarchy and cftypes.  While this worked, it added a lot of double
lockings and was generally cumbersome.

kernfs provides a mechanism to opt out of active protection and cgroup
was already using it for removal and subtree_control.  There's no
reason to mix both methods of avoiding circular locking dependency and
the preceding cgroup_kn_lock_live() changes applied it to all relevant
cgroup kernfs operations making it unnecessary to nest cgroup_mutex
under kernfs active protection.  The previous patch reversed the
original lock ordering and put cgroup_mutex above kernfs active
protection.

After these changes, all cgroup_tree_mutex usages are now accompanied
by cgroup_mutex making the former completely redundant.  This patch
removes cgroup_tree_mutex and all its usages.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
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@htejun
htejun committed May 13, 2014
1 parent 01f6474 commit 8353da1
Showing 1 changed file with 9 additions and 55 deletions.
64 changes: 9 additions & 55 deletions kernel/cgroup.c
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Original file line number Diff line number Diff line change
Expand Up @@ -70,15 +70,6 @@
#define CGROUP_FILE_NAME_MAX (MAX_CGROUP_TYPE_NAMELEN + \
MAX_CFTYPE_NAME + 2)

/*
* cgroup_tree_mutex nests above cgroup_mutex and protects cftypes, file
* creation/removal and hierarchy changing operations including cgroup
* creation, removal, css association and controller rebinding. This outer
* lock is needed mainly to resolve the circular dependency between kernfs
* active ref and cgroup_mutex. cgroup_tree_mutex nests above both.
*/
static DEFINE_MUTEX(cgroup_tree_mutex);

/*
* cgroup_mutex is the master lock. Any modification to cgroup or its
* hierarchy must be performed while holding it.
Expand Down Expand Up @@ -111,11 +102,10 @@ static DEFINE_SPINLOCK(cgroup_idr_lock);
*/
static DEFINE_SPINLOCK(release_agent_path_lock);

#define cgroup_assert_mutexes_or_rcu_locked() \
#define cgroup_assert_mutex_or_rcu_locked() \
rcu_lockdep_assert(rcu_read_lock_held() || \
lockdep_is_held(&cgroup_tree_mutex) || \
lockdep_is_held(&cgroup_mutex), \
"cgroup_[tree_]mutex or RCU read lock required");
"cgroup_mutex or RCU read lock required");

/*
* cgroup destruction makes heavy use of work items and there can be a lot
Expand Down Expand Up @@ -243,7 +233,6 @@ static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
{
if (ss)
return rcu_dereference_check(cgrp->subsys[ss->id],
lockdep_is_held(&cgroup_tree_mutex) ||
lockdep_is_held(&cgroup_mutex));
else
return &cgrp->dummy_css;
Expand Down Expand Up @@ -347,7 +336,6 @@ static int notify_on_release(const struct cgroup *cgrp)
for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++) \
if (!((css) = rcu_dereference_check( \
(cgrp)->subsys[(ssid)], \
lockdep_is_held(&cgroup_tree_mutex) || \
lockdep_is_held(&cgroup_mutex)))) { } \
else

Expand Down Expand Up @@ -381,7 +369,7 @@ static int notify_on_release(const struct cgroup *cgrp)
/* iterate over child cgrps, lock should be held throughout iteration */
#define cgroup_for_each_live_child(child, cgrp) \
list_for_each_entry((child), &(cgrp)->children, sibling) \
if (({ lockdep_assert_held(&cgroup_tree_mutex); \
if (({ lockdep_assert_held(&cgroup_mutex); \
cgroup_is_dead(child); })) \
; \
else
Expand Down Expand Up @@ -869,7 +857,6 @@ static void cgroup_destroy_root(struct cgroup_root *root)
struct cgroup *cgrp = &root->cgrp;
struct cgrp_cset_link *link, *tmp_link;

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

BUG_ON(atomic_read(&root->nr_cgrps));
Expand Down Expand Up @@ -899,7 +886,6 @@ static void cgroup_destroy_root(struct cgroup_root *root)
cgroup_exit_root_id(root);

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

kernfs_destroy_root(root->kf_root);
cgroup_free_root(root);
Expand Down Expand Up @@ -1096,7 +1082,6 @@ static void cgroup_kn_unlock(struct kernfs_node *kn)
cgrp = kn->parent->priv;

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

kernfs_unbreak_active_protection(kn);
cgroup_put(cgrp);
Expand Down Expand Up @@ -1135,7 +1120,6 @@ static struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn)
cgroup_get(cgrp);
kernfs_break_active_protection(kn);

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

if (!cgroup_is_dead(cgrp))
Expand All @@ -1149,7 +1133,6 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
{
char name[CGROUP_FILE_NAME_MAX];

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);
kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
}
Expand Down Expand Up @@ -1179,7 +1162,6 @@ static int rebind_subsystems(struct cgroup_root *dst_root, unsigned int ss_mask)
struct cgroup_subsys *ss;
int ssid, i, ret;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

for_each_subsys(ss, ssid) {
Expand Down Expand Up @@ -1457,7 +1439,6 @@ static int cgroup_remount(struct kernfs_root *kf_root, int *flags, char *data)
return -EINVAL;
}

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

/* See what subsystems are wanted */
Expand Down Expand Up @@ -1503,7 +1484,6 @@ static int cgroup_remount(struct kernfs_root *kf_root, int *flags, char *data)
kfree(opts.release_agent);
kfree(opts.name);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_tree_mutex);
return ret;
}

Expand Down Expand Up @@ -1606,7 +1586,6 @@ static int cgroup_setup_root(struct cgroup_root *root, unsigned int ss_mask)
struct css_set *cset;
int i, ret;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

ret = cgroup_idr_alloc(&root->cgroup_idr, root_cgrp, 1, 2, GFP_NOWAIT);
Expand Down Expand Up @@ -1696,7 +1675,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
if (!use_task_css_set_links)
cgroup_enable_task_cg_lists();

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

/* First find the desired set of subsystems */
Expand Down Expand Up @@ -1761,9 +1739,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
*/
if (!atomic_inc_not_zero(&root->cgrp.refcnt)) {
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_tree_mutex);
msleep(10);
mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
goto retry;
}
Expand Down Expand Up @@ -1796,7 +1772,6 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,

out_unlock:
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_tree_mutex);

kfree(opts.release_agent);
kfree(opts.name);
Expand Down Expand Up @@ -2507,7 +2482,6 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
struct css_set *src_cset;
int ret;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

/* look up all csses currently attached to @cgrp's subtree */
Expand Down Expand Up @@ -2866,20 +2840,18 @@ static int cgroup_rename(struct kernfs_node *kn, struct kernfs_node *new_parent,
return -EPERM;

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

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

ret = kernfs_rename(kn, new_parent, new_name_str);

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

kernfs_unbreak_active_protection(kn);
kernfs_unbreak_active_protection(new_parent);
Expand Down Expand Up @@ -2944,7 +2916,6 @@ static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
struct cftype *cft;
int ret;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

for (cft = cfts; cft->name[0] != '\0'; cft++) {
Expand Down Expand Up @@ -2980,7 +2951,6 @@ static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
struct cgroup_subsys_state *css;
int ret = 0;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

/* add/rm files for all cgroups created before */
Expand Down Expand Up @@ -3049,7 +3019,6 @@ static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)

static int cgroup_rm_cftypes_locked(struct cftype *cfts)
{
lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

if (!cfts || !cfts[0].ss)
Expand All @@ -3076,11 +3045,9 @@ int cgroup_rm_cftypes(struct cftype *cfts)
{
int ret;

mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
ret = cgroup_rm_cftypes_locked(cfts);
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_tree_mutex);
return ret;
}

Expand Down Expand Up @@ -3109,7 +3076,6 @@ int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
if (ret)
return ret;

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

list_add_tail(&cfts->node, &ss->cfts);
Expand All @@ -3118,7 +3084,6 @@ int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
cgroup_rm_cftypes_locked(cfts);

mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_tree_mutex);
return ret;
}

Expand Down Expand Up @@ -3158,7 +3123,7 @@ css_next_child(struct cgroup_subsys_state *pos_css,
struct cgroup *cgrp = parent_css->cgroup;
struct cgroup *next;

cgroup_assert_mutexes_or_rcu_locked();
cgroup_assert_mutex_or_rcu_locked();

/*
* @pos could already have been removed. Once a cgroup is removed,
Expand Down Expand Up @@ -3224,7 +3189,7 @@ css_next_descendant_pre(struct cgroup_subsys_state *pos,
{
struct cgroup_subsys_state *next;

cgroup_assert_mutexes_or_rcu_locked();
cgroup_assert_mutex_or_rcu_locked();

/* if first iteration, visit @root */
if (!pos)
Expand Down Expand Up @@ -3264,7 +3229,7 @@ css_rightmost_descendant(struct cgroup_subsys_state *pos)
{
struct cgroup_subsys_state *last, *tmp;

cgroup_assert_mutexes_or_rcu_locked();
cgroup_assert_mutex_or_rcu_locked();

do {
last = pos;
Expand Down Expand Up @@ -3311,7 +3276,7 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
{
struct cgroup_subsys_state *next;

cgroup_assert_mutexes_or_rcu_locked();
cgroup_assert_mutex_or_rcu_locked();

/* if first iteration, visit leftmost descendant which may be @root */
if (!pos)
Expand Down Expand Up @@ -4178,7 +4143,6 @@ static int online_css(struct cgroup_subsys_state *css)
struct cgroup_subsys *ss = css->ss;
int ret = 0;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

if (ss->css_online)
Expand All @@ -4196,7 +4160,6 @@ static void offline_css(struct cgroup_subsys_state *css)
{
struct cgroup_subsys *ss = css->ss;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

if (!(css->flags & CSS_ONLINE))
Expand Down Expand Up @@ -4399,7 +4362,6 @@ static void css_killed_work_fn(struct work_struct *work)
container_of(work, struct cgroup_subsys_state, destroy_work);
struct cgroup *cgrp = css->cgroup;

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

/*
Expand All @@ -4417,7 +4379,6 @@ static void css_killed_work_fn(struct work_struct *work)
cgroup_destroy_css_killed(cgrp);

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

/*
* Put the css refs from kill_css(). Each css holds an extra
Expand Down Expand Up @@ -4450,7 +4411,6 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
*/
static void kill_css(struct cgroup_subsys_state *css)
{
lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

/*
Expand Down Expand Up @@ -4510,7 +4470,6 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
bool empty;
int ssid;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

/*
Expand Down Expand Up @@ -4593,7 +4552,6 @@ static void cgroup_destroy_css_killed(struct cgroup *cgrp)
{
struct cgroup *parent = cgrp->parent;

lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

/* delete this cgroup from parent->children */
Expand Down Expand Up @@ -4647,7 +4605,6 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)

printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);

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

idr_init(&ss->css_idr);
Expand Down Expand Up @@ -4685,7 +4642,6 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
cgrp_dfl_root.subsys_mask |= 1 << ss->id;

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

/**
Expand Down Expand Up @@ -4735,7 +4691,6 @@ int __init cgroup_init(void)

BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));

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

/* Add init_css_set to the hash table */
Expand All @@ -4745,7 +4700,6 @@ int __init cgroup_init(void)
BUG_ON(cgroup_setup_root(&cgrp_dfl_root, 0));

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

for_each_subsys(ss, ssid) {
if (ss->early_init) {
Expand Down

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