cgroup: introduce cgroup_tree_mutex
Currently cgroup uses combination of inode->i_mutex'es and
cgroup_mutex for synchronization. With the scheduled kernfs
conversion, i_mutex'es will be removed. Unfortunately, just using
cgroup_mutex isn't possible. All kernfs file and syscall operations,
most of which require grabbing cgroup_mutex, will be called with
kernfs active ref held and, if we try to perform kernfs removals under
cgroup_mutex, it can deadlock as kernfs_remove() tries to drain the
target node.
Let's introduce a new outer mutex, cgroup_tree_mutex, which protects
stuff used during hierarchy changing operations - cftypes and all the
operations which may affect the cgroupfs. It also covers css
association and iteration. This allows cgroup_css(), for_each_css()
and other css iterators to be called under cgroup_tree_mutex. The new
mutex will nest above both kernfs's active ref protection and
cgroup_mutex. By protecting tree modifications with a separate outer
mutex, we can get rid of the forementioned deadlock condition.
Actual file additions and removals now require cgroup_tree_mutex
instead of cgroup_mutex. Currently, cgroup_tree_mutex is never used
without cgroup_mutex; however, we'll soon add hierarchy modification
sections which are only protected by cgroup_tree_mutex. In the
future, we might want to make the locking more granular by better
splitting the coverages of the two mutexes. For now, this should do.
v2: Rebased on top of 0ab02ca8f887 ("cgroup: protect modifications to
cgroup_idr with cgroup_mutex").
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index fc2db07..cb20d12 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -68,6 +68,15 @@
#define CGROUP_PIDLIST_DESTROY_DELAY HZ
/*
+ * 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.
*/
@@ -84,10 +93,11 @@
*/
static DEFINE_SPINLOCK(release_agent_path_lock);
-#define cgroup_assert_mutex_or_rcu_locked() \
+#define cgroup_assert_mutexes_or_rcu_locked() \
rcu_lockdep_assert(rcu_read_lock_held() || \
+ lockdep_is_held(&cgroup_tree_mutex) || \
lockdep_is_held(&cgroup_mutex), \
- "cgroup_mutex or RCU read lock required");
+ "cgroup_[tree_]mutex or RCU read lock required");
/*
* cgroup destruction makes heavy use of work items and there can be a lot
@@ -179,7 +189,8 @@
{
if (ss)
return rcu_dereference_check(cgrp->subsys[ss->id],
- lockdep_is_held(&cgroup_mutex));
+ lockdep_is_held(&cgroup_tree_mutex) ||
+ lockdep_is_held(&cgroup_mutex));
else
return &cgrp->dummy_css;
}
@@ -235,6 +246,7 @@
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
@@ -883,7 +895,7 @@
struct cfent *cfe;
lockdep_assert_held(&cgrp->dentry->d_inode->i_mutex);
- lockdep_assert_held(&cgroup_mutex);
+ lockdep_assert_held(&cgroup_tree_mutex);
/*
* If we're doing cleanup due to failure of cgroup_create(),
@@ -948,7 +960,8 @@
struct cgroup_subsys *ss;
int i, ret;
- BUG_ON(!mutex_is_locked(&cgroup_mutex));
+ lockdep_assert_held(&cgroup_tree_mutex);
+ lockdep_assert_held(&cgroup_mutex);
/* Check that any added subsystems are currently free */
for_each_subsys(ss, i)
@@ -1220,6 +1233,7 @@
}
mutex_lock(&cgrp->dentry->d_inode->i_mutex);
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
/* See what subsystems are wanted */
@@ -1263,6 +1277,7 @@
kfree(opts.release_agent);
kfree(opts.name);
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
return ret;
}
@@ -1494,6 +1509,7 @@
inode = sb->s_root->d_inode;
mutex_lock(&inode->i_mutex);
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
ret = idr_alloc(&root->cgroup_idr, root_cgrp, 0, 1, GFP_KERNEL);
@@ -1568,6 +1584,7 @@
BUG_ON(root->number_of_cgroups != 1);
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&inode->i_mutex);
} else {
/*
@@ -1598,6 +1615,7 @@
unlock_drop:
cgroup_exit_root_id(root);
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&inode->i_mutex);
drop_new_super:
deactivate_locked_super(sb);
@@ -1620,6 +1638,7 @@
BUG_ON(!list_empty(&cgrp->children));
mutex_lock(&cgrp->dentry->d_inode->i_mutex);
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
/* Rebind all subsystems back to the default hierarchy */
@@ -1650,6 +1669,7 @@
cgroup_exit_root_id(root);
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
simple_xattrs_free(&cgrp->xattrs);
@@ -2625,7 +2645,7 @@
int ret;
lockdep_assert_held(&cgrp->dentry->d_inode->i_mutex);
- lockdep_assert_held(&cgroup_mutex);
+ lockdep_assert_held(&cgroup_tree_mutex);
for (cft = cfts; cft->name[0] != '\0'; cft++) {
/* does cft->flags tell us to skip this file on @cgrp? */
@@ -2659,6 +2679,7 @@
* Instead, we use css_for_each_descendant_pre() and drop RCU read
* lock before calling cgroup_addrm_files().
*/
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
}
@@ -2679,6 +2700,7 @@
if (!cfts || ss->root == &cgroup_dummy_root ||
!atomic_inc_not_zero(&sb->s_active)) {
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
return 0;
}
@@ -2702,7 +2724,9 @@
prev = cgrp->dentry;
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_lock(&inode->i_mutex);
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
if (cgrp->serial_nr < update_before && !cgroup_is_dead(cgrp))
ret = cgroup_addrm_files(cgrp, cfts, is_add);
@@ -2711,6 +2735,7 @@
break;
}
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
dput(prev);
deactivate_super(sb);
return ret;
@@ -2856,7 +2881,7 @@
struct cgroup *cgrp = parent_css->cgroup;
struct cgroup *next;
- cgroup_assert_mutex_or_rcu_locked();
+ cgroup_assert_mutexes_or_rcu_locked();
/*
* @pos could already have been removed. Once a cgroup is removed,
@@ -2914,7 +2939,7 @@
{
struct cgroup_subsys_state *next;
- cgroup_assert_mutex_or_rcu_locked();
+ cgroup_assert_mutexes_or_rcu_locked();
/* if first iteration, visit @root */
if (!pos)
@@ -2955,7 +2980,7 @@
{
struct cgroup_subsys_state *last, *tmp;
- cgroup_assert_mutex_or_rcu_locked();
+ cgroup_assert_mutexes_or_rcu_locked();
do {
last = pos;
@@ -3003,7 +3028,7 @@
{
struct cgroup_subsys_state *next;
- cgroup_assert_mutex_or_rcu_locked();
+ cgroup_assert_mutexes_or_rcu_locked();
/* if first iteration, visit leftmost descendant which may be @root */
if (!pos)
@@ -3977,6 +4002,7 @@
struct cgroup_subsys *ss = css->ss;
int ret = 0;
+ lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);
if (ss->css_online)
@@ -3994,6 +4020,7 @@
{
struct cgroup_subsys *ss = css->ss;
+ lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);
if (!(css->flags & CSS_ONLINE))
@@ -4093,6 +4120,8 @@
}
rcu_assign_pointer(cgrp->name, name);
+ mutex_lock(&cgroup_tree_mutex);
+
/*
* Only live parents can have children. Note that the liveliness
* check isn't strictly necessary because cgroup_mkdir() and
@@ -4102,7 +4131,7 @@
*/
if (!cgroup_lock_live_group(parent)) {
err = -ENODEV;
- goto err_free_name;
+ goto err_unlock_tree;
}
/*
@@ -4176,6 +4205,7 @@
}
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
return 0;
@@ -4186,7 +4216,8 @@
deactivate_super(sb);
err_unlock:
mutex_unlock(&cgroup_mutex);
-err_free_name:
+err_unlock_tree:
+ mutex_unlock(&cgroup_tree_mutex);
kfree(rcu_dereference_raw(cgrp->name));
err_free_cgrp:
kfree(cgrp);
@@ -4195,6 +4226,7 @@
err_destroy:
cgroup_destroy_locked(cgrp);
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&dentry->d_inode->i_mutex);
return err;
}
@@ -4217,6 +4249,7 @@
container_of(work, struct cgroup_subsys_state, destroy_work);
struct cgroup *cgrp = css->cgroup;
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
/*
@@ -4234,6 +4267,7 @@
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
@@ -4321,6 +4355,7 @@
int ssid;
lockdep_assert_held(&d->d_inode->i_mutex);
+ lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);
/*
@@ -4407,6 +4442,7 @@
struct cgroup *parent = cgrp->parent;
struct dentry *d = cgrp->dentry;
+ lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);
/* delete this cgroup from parent->children */
@@ -4422,9 +4458,11 @@
{
int ret;
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
ret = cgroup_destroy_locked(dentry->d_fsdata);
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
return ret;
}
@@ -4454,6 +4492,7 @@
printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
/* init base cftset */
@@ -4482,6 +4521,7 @@
BUG_ON(online_css(css));
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
}
/**
@@ -5021,7 +5061,7 @@
{
struct cgroup *cgrp;
- cgroup_assert_mutex_or_rcu_locked();
+ cgroup_assert_mutexes_or_rcu_locked();
cgrp = idr_find(&ss->root->cgroup_idr, id);
if (cgrp)