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cgroup: deactivate CSS's and mark cgroup dead before invoking ->pre_destroy() 

Because ->pre_destroy() could fail and can't be called under
cgroup_mutex, cgroup destruction did something very ugly.

  1. Grab cgroup_mutex and verify it can be destroyed; fail otherwise.

  2. Release cgroup_mutex and call ->pre_destroy().

  3. Re-grab cgroup_mutex and verify it can still be destroyed; fail
     otherwise.

  4. Continue destroying.

In addition to being ugly, it has been always broken in various ways.
For example, memcg ->pre_destroy() expects the cgroup to be inactive
after it's done but tasks can be attached and detached between #2 and
#3 and the conditions that memcg verified in ->pre_destroy() might no
longer hold by the time control reaches #3.

Now that ->pre_destroy() is no longer allowed to fail.  We can switch
to the following.

  1. Grab cgroup_mutex and verify it can be destroyed; fail otherwise.

  2. Deactivate CSS's and mark the cgroup removed thus preventing any
     further operations which can invalidate the verification from #1.

  3. Release cgroup_mutex and call ->pre_destroy().

  4. Re-grab cgroup_mutex and continue destroying.

After this change, controllers can safely assume that ->pre_destroy()
will only be called only once for a given cgroup and, once
->pre_destroy() is called, the cgroup will stay dormant till it's
destroyed.

This removes the only reason ->pre_destroy() can fail - new task being
attached or child cgroup being created inbetween.  Error out path is
removed and ->pre_destroy() invocation is open coded in
cgroup_rmdir().

v2: cgroup_call_pre_destroy() removal moved to this patch per Michal.
    Commit message updated per Glauber.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Glauber Costa <glommer@parallels.com>
This commit is contained in:
Tejun Heo 2012-11-05 09:16:59 -08:00
parent 976c06bccc
commit 1a90dd508b
1 changed files with 19 additions and 46 deletions
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65
kernel/cgroup.c
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@ -851,27 +851,6 @@ static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
return inode;
}
/*
* Call subsys's pre_destroy handler.
* This is called before css refcnt check.
*/
static int cgroup_call_pre_destroy(struct cgroup *cgrp)
{
struct cgroup_subsys *ss;
int ret = 0;
for_each_subsys(cgrp->root, ss) {
if (!ss->pre_destroy)
continue;
ret = ss->pre_destroy(cgrp);
if (WARN_ON_ONCE(ret))
break;
}
return ret;
}
static void cgroup_diput(struct dentry *dentry, struct inode *inode)
{
/* is dentry a directory ? if so, kfree() associated cgroup */
@ -4078,19 +4057,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
DEFINE_WAIT(wait);
struct cgroup_event *event, *tmp;
struct cgroup_subsys *ss;
int ret;
/* the vfs holds both inode->i_mutex already */
mutex_lock(&cgroup_mutex);
if (atomic_read(&cgrp->count) != 0) {
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}
if (!list_empty(&cgrp->children)) {
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}
mutex_unlock(&cgroup_mutex);
/*
* In general, subsystem has no css->refcnt after pre_destroy(). But
@ -4103,16 +4069,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
*/
set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
/*
* Call pre_destroy handlers of subsys. Notify subsystems
* that rmdir() request comes.
*/
ret = cgroup_call_pre_destroy(cgrp);
if (ret) {
clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
return ret;
}
/* the vfs holds both inode->i_mutex already */
mutex_lock(&cgroup_mutex);
parent = cgrp->parent;
if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
@ -4122,13 +4079,30 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
}
prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE);
/* block new css_tryget() by deactivating refcnt */
/*
* Block new css_tryget() by deactivating refcnt and mark @cgrp
* removed. This makes future css_tryget() and child creation
* attempts fail thus maintaining the removal conditions verified
* above.
*/
for_each_subsys(cgrp->root, ss) {
struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
WARN_ON(atomic_read(&css->refcnt) < 0);
atomic_add(CSS_DEACT_BIAS, &css->refcnt);
}
set_bit(CGRP_REMOVED, &cgrp->flags);
/*
* Tell subsystems to initate destruction. pre_destroy() should be
* called with cgroup_mutex unlocked. See 3fa59dfbc3 ("cgroup: fix
* potential deadlock in pre_destroy") for details.
*/
mutex_unlock(&cgroup_mutex);
for_each_subsys(cgrp->root, ss)
if (ss->pre_destroy)
WARN_ON_ONCE(ss->pre_destroy(cgrp));
mutex_lock(&cgroup_mutex);
/*
* Put all the base refs. Each css holds an extra reference to the
@ -4144,7 +4118,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
raw_spin_lock(&release_list_lock);
set_bit(CGRP_REMOVED, &cgrp->flags);
if (!list_empty(&cgrp->release_list))
list_del_init(&cgrp->release_list);
raw_spin_unlock(&release_list_lock);