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Subject[PATCH 02/13] cgroup: introduce cgroup_tree_mutex
Date
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.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/cgroup.c | 66 +++++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 53 insertions(+), 13 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 57eb942..e41987d 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_MUTEX(cgroup_mutex);
*/
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 @@ static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
{
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 @@ 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

@@ -881,7 +893,7 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
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(),
@@ -946,7 +958,8 @@ static int rebind_subsystems(struct cgroupfs_root *root,
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)
@@ -1218,6 +1231,7 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
}

mutex_lock(&cgrp->dentry->d_inode->i_mutex);
+ mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);

/* See what subsystems are wanted */
@@ -1261,6 +1275,7 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
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;
}
@@ -1492,6 +1507,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
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);
@@ -1566,6 +1582,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
BUG_ON(root->number_of_cgroups != 1);

mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&inode->i_mutex);
} else {
/*
@@ -1596,6 +1613,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
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);
@@ -1618,6 +1636,7 @@ static void cgroup_kill_sb(struct super_block *sb)
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 */
@@ -1648,6 +1667,7 @@ static void cgroup_kill_sb(struct super_block *sb)
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);
@@ -2623,7 +2643,7 @@ static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
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? */
@@ -2657,6 +2677,7 @@ static void cgroup_cfts_prepare(void)
* 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);
}

@@ -2677,6 +2698,7 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
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;
}

@@ -2700,7 +2722,9 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
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);
@@ -2709,6 +2733,7 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
break;
}
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
dput(prev);
deactivate_super(sb);
return ret;
@@ -2854,7 +2879,7 @@ css_next_child(struct cgroup_subsys_state *pos_css,
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,
@@ -2912,7 +2937,7 @@ css_next_descendant_pre(struct cgroup_subsys_state *pos,
{
struct cgroup_subsys_state *next;

- cgroup_assert_mutex_or_rcu_locked();
+ cgroup_assert_mutexes_or_rcu_locked();

/* if first iteration, visit @root */
if (!pos)
@@ -2953,7 +2978,7 @@ css_rightmost_descendant(struct cgroup_subsys_state *pos)
{
struct cgroup_subsys_state *last, *tmp;

- cgroup_assert_mutex_or_rcu_locked();
+ cgroup_assert_mutexes_or_rcu_locked();

do {
last = pos;
@@ -3001,7 +3026,7 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
{
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)
@@ -3975,6 +4000,7 @@ 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)
@@ -3992,6 +4018,7 @@ 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))
@@ -4101,6 +4128,8 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
goto err_free_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
@@ -4110,7 +4139,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
*/
if (!cgroup_lock_live_group(parent)) {
err = -ENODEV;
- goto err_free_id;
+ goto err_unlock_tree;
}

/* Grab a reference on the superblock so the hierarchy doesn't
@@ -4174,6 +4203,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
}

mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&cgrp->dentry->d_inode->i_mutex);

return 0;
@@ -4182,7 +4212,8 @@ err_unlock:
mutex_unlock(&cgroup_mutex);
/* Release the reference count that we took on the superblock */
deactivate_super(sb);
-err_free_id:
+err_unlock_tree:
+ mutex_unlock(&cgroup_tree_mutex);
idr_remove(&root->cgroup_idr, cgrp->id);
err_free_name:
kfree(rcu_dereference_raw(cgrp->name));
@@ -4193,6 +4224,7 @@ err_free_cgrp:
err_destroy:
cgroup_destroy_locked(cgrp);
mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
mutex_unlock(&dentry->d_inode->i_mutex);
return err;
}
@@ -4215,6 +4247,7 @@ 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);

/*
@@ -4232,6 +4265,7 @@ 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
@@ -4319,6 +4353,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
int ssid;

lockdep_assert_held(&d->d_inode->i_mutex);
+ lockdep_assert_held(&cgroup_tree_mutex);
lockdep_assert_held(&cgroup_mutex);

/*
@@ -4405,6 +4440,7 @@ static void cgroup_destroy_css_killed(struct cgroup *cgrp)
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 */
@@ -4420,9 +4456,11 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
{
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;
}
@@ -4452,6 +4490,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)

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

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

/* init base cftset */
@@ -4480,6 +4519,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
BUG_ON(online_css(css));

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

/**
@@ -5019,7 +5059,7 @@ struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
{
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)
--
1.8.5.3


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