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cgroup: no need to check css refs for release notification 

We no longer fail rmdir() when there're still css refs, so we don't
need to check css refs in check_for_release().

This also voids a bug. cgroup_has_css_refs() accesses subsys[i]
without cgroup_mutex, so it can race with cgroup_unload_subsys().

cgroup_has_css_refs()
...
  if (ss == NULL || ss->root != cgrp->root)

if ss pointers to net_cls_subsys, and cls_cgroup module is unloaded
right after the former check but before the latter, the memory that
net_cls_subsys resides has become invalid.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Li Zefan 2013-03-01 15:06:07 +08:00 committed by Tejun Heo
parent f440d98f8e
commit f50daa704f
1 changed files with 8 additions and 59 deletions
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67
kernel/cgroup.c
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@ -4341,47 +4341,6 @@ static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
return cgroup_create(c_parent, dentry, mode | S_IFDIR);
}
/*
* Check the reference count on each subsystem. Since we already
* established that there are no tasks in the cgroup, if the css refcount
* is also 1, then there should be no outstanding references, so the
* subsystem is safe to destroy. We scan across all subsystems rather than
* using the per-hierarchy linked list of mounted subsystems since we can
* be called via check_for_release() with no synchronization other than
* RCU, and the subsystem linked list isn't RCU-safe.
*/
static int cgroup_has_css_refs(struct cgroup *cgrp)
{
int i;
/*
* We won't need to lock the subsys array, because the subsystems
* we're concerned about aren't going anywhere since our cgroup root
* has a reference on them.
*/
for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
struct cgroup_subsys *ss = subsys[i];
struct cgroup_subsys_state *css;
/* Skip subsystems not present or not in this hierarchy */
if (ss == NULL || ss->root != cgrp->root)
continue;
css = cgrp->subsys[ss->subsys_id];
/*
* When called from check_for_release() it's possible
* that by this point the cgroup has been removed
* and the css deleted. But a false-positive doesn't
* matter, since it can only happen if the cgroup
* has been deleted and hence no longer needs the
* release agent to be called anyway.
*/
if (css && css_refcnt(css) > 1)
return 1;
}
return 0;
}
static int cgroup_destroy_locked(struct cgroup *cgrp)
__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
{
@ -5108,12 +5067,15 @@ static void check_for_release(struct cgroup *cgrp)
{
/* All of these checks rely on RCU to keep the cgroup
* structure alive */
if (cgroup_is_releasable(cgrp) && !atomic_read(&cgrp->count)
&& list_empty(&cgrp->children) && !cgroup_has_css_refs(cgrp)) {
/* Control Group is currently removeable. If it's not
if (cgroup_is_releasable(cgrp) &&
!atomic_read(&cgrp->count) && list_empty(&cgrp->children)) {
/*
* Control Group is currently removeable. If it's not
* already queued for a userspace notification, queue
* it now */
* it now
*/
int need_schedule_work = 0;
raw_spin_lock(&release_list_lock);
if (!cgroup_is_removed(cgrp) &&
list_empty(&cgrp->release_list)) {
@ -5146,24 +5108,11 @@ EXPORT_SYMBOL_GPL(__css_tryget);
/* Caller must verify that the css is not for root cgroup */
void __css_put(struct cgroup_subsys_state *css)
{
struct cgroup *cgrp = css->cgroup;
int v;
rcu_read_lock();
v = css_unbias_refcnt(atomic_dec_return(&css->refcnt));
switch (v) {
case 1:
if (notify_on_release(cgrp)) {
set_bit(CGRP_RELEASABLE, &cgrp->flags);
check_for_release(cgrp);
}
break;
case 0:
if (v == 0)
schedule_work(&css->dput_work);
break;
}
rcu_read_unlock();
}
EXPORT_SYMBOL_GPL(__css_put);