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cgroup: remove cgroupfs_root->refcnt

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Currently, cgroupfs_root and its ->top_cgroup are separated reference
counted and the latter's is ignored.  There's no reason to do this
separately.  This patch removes cgroupfs_root->refcnt and destroys
cgroupfs_root when the top_cgroup is released.

* cgroup_put() updated to ignore cgroup_is_dead() test for top
  cgroups.  cgroup_free_fn() updated to handle root destruction when
  releasing a top cgroup.

* As root destruction is now bounced through cgroup destruction, it is
  asynchronous.  Update cgroup_mount() so that it waits for pending
  release which is currently implemented using msleep().  Converting
  this to proper wait_queue isn't hard but likely unnecessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
This commit is contained in:
Tejun Heo 2014-02-12 09:29:50 -05:00
parent 3c9c825b8b
commit 776f02fa4e
2 changed files with 39 additions and 51 deletions
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4
include/linux/cgroup.h
View file

@ -282,12 +282,10 @@ struct cgroupfs_root {
/* The bitmask of subsystems attached to this hierarchy */
unsigned long subsys_mask;
atomic_t refcnt;
/* Unique id for this hierarchy. */
int hierarchy_id;
/* The root cgroup for this hierarchy */
/* The root cgroup. Root is destroyed on its release. */
struct cgroup top_cgroup;
/* Number of cgroups in the hierarchy, used only for /proc/cgroups */

86
kernel/cgroup.c
View file

@ -53,6 +53,7 @@
#include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */
#include <linux/flex_array.h> /* used in cgroup_attach_task */
#include <linux/kthread.h>
#include <linux/delay.h>
#include <linux/atomic.h>
@ -728,37 +729,16 @@ static void cgroup_free_root(struct cgroupfs_root *root)
}
}
static void cgroup_get_root(struct cgroupfs_root *root)
{
/*
* The caller must ensure that @root is alive, which can be
* achieved by holding a ref on one of the member cgroups or
* following a registered reference to @root while holding
* cgroup_tree_mutex.
*/
WARN_ON_ONCE(atomic_read(&root->refcnt) <= 0);
atomic_inc(&root->refcnt);
}
static void cgroup_put_root(struct cgroupfs_root *root)
static void cgroup_destroy_root(struct cgroupfs_root *root)
{
struct cgroup *cgrp = &root->top_cgroup;
struct cgrp_cset_link *link, *tmp_link;
int ret;
/*
* @root's refcnt reaching zero and its deregistration should be
* atomic w.r.t. cgroup_tree_mutex. This ensures that
* cgroup_get_root() is safe to invoke if @root is registered.
*/
mutex_lock(&cgroup_tree_mutex);
if (!atomic_dec_and_test(&root->refcnt)) {
mutex_unlock(&cgroup_tree_mutex);
return;
}
mutex_lock(&cgroup_mutex);
BUG_ON(atomic_read(&root->nr_cgrps) != 1);
BUG_ON(atomic_read(&root->nr_cgrps));
BUG_ON(!list_empty(&cgrp->children));
/* Rebind all subsystems back to the default hierarchy */
@ -929,21 +909,24 @@ static void cgroup_free_fn(struct work_struct *work)
struct cgroup *cgrp = container_of(work, struct cgroup, destroy_work);
atomic_dec(&cgrp->root->nr_cgrps);
/*
* We get a ref to the parent, and put the ref when this cgroup is
* being freed, so it's guaranteed that the parent won't be
* destroyed before its children.
*/
cgroup_put(cgrp->parent);
/* put the root reference that we took when we created the cgroup */
cgroup_put_root(cgrp->root);
cgroup_pidlist_destroy_all(cgrp);
kernfs_put(cgrp->kn);
kfree(cgrp);
if (cgrp->parent) {
/*
* We get a ref to the parent, and put the ref when this
* cgroup is being freed, so it's guaranteed that the
* parent won't be destroyed before its children.
*/
cgroup_put(cgrp->parent);
kernfs_put(cgrp->kn);
kfree(cgrp);
} else {
/*
* This is top cgroup's refcnt reaching zero, which
* indicates that the root should be released.
*/
cgroup_destroy_root(cgrp->root);
}
}
static void cgroup_free_rcu(struct rcu_head *head)
@ -965,7 +948,7 @@ static void cgroup_put(struct cgroup *cgrp)
{
if (!atomic_dec_and_test(&cgrp->refcnt))
return;
if (WARN_ON_ONCE(!cgroup_is_dead(cgrp)))
if (WARN_ON_ONCE(cgrp->parent && !cgroup_is_dead(cgrp)))
return;
/*
@ -1356,7 +1339,6 @@ static void init_cgroup_root(struct cgroupfs_root *root)
{
struct cgroup *cgrp = &root->top_cgroup;
atomic_set(&root->refcnt, 1);
INIT_LIST_HEAD(&root->root_list);
atomic_set(&root->nr_cgrps, 1);
cgrp->root = root;
@ -1485,7 +1467,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
struct cgroup_sb_opts opts;
struct dentry *dentry;
int ret;
retry:
mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
@ -1531,7 +1513,21 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
}
}
cgroup_get_root(root);
/*
* A root's lifetime is governed by its top cgroup. Zero
* ref indicate that the root is being destroyed. Wait for
* destruction to complete so that the subsystems are free.
* We can use wait_queue for the wait but this path is
* super cold. Let's just sleep for a bit and retry.
*/
if (!atomic_inc_not_zero(&root->top_cgroup.refcnt)) {
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_tree_mutex);
msleep(10);
goto retry;
}
ret = 0;
goto out_unlock;
}
@ -1558,7 +1554,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
dentry = kernfs_mount(fs_type, flags, root->kf_root);
if (IS_ERR(dentry))
cgroup_put_root(root);
cgroup_put(&root->top_cgroup);
return dentry;
}
@ -1567,7 +1563,7 @@ static void cgroup_kill_sb(struct super_block *sb)
struct kernfs_root *kf_root = kernfs_root_from_sb(sb);
struct cgroupfs_root *root = cgroup_root_from_kf(kf_root);
cgroup_put_root(root);
cgroup_put(&root->top_cgroup);
kernfs_kill_sb(sb);
}
@ -3708,12 +3704,6 @@ static long cgroup_create(struct cgroup *parent, const char *name,
/* allocation complete, commit to creation */
list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
atomic_inc(&root->nr_cgrps);
/*
* Grab a reference on the root and parent so that they don't get
* deleted while there are child cgroups.
*/
cgroup_get_root(root);
cgroup_get(parent);
/*