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 fail if it can't 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.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
kernel/cgroup.c | 41 +++++++++++++++++++----------------------
1 file changed, 19 insertions(+), 22 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b3010ae..66204a6 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4058,18 +4058,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
struct cgroup_event *event, *tmp;
struct cgroup_subsys *ss;
- /* 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
* in racy cases, subsystem may have to get css->refcnt after
@@ -4081,14 +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.
- */
- for_each_subsys(cgrp->root, ss)
- if (ss->pre_destroy)
- WARN_ON_ONCE(ss->pre_destroy(cgrp));
-
+ /* the vfs holds both inode->i_mutex already */
mutex_lock(&cgroup_mutex);
parent = cgrp->parent;
if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
@@ -4098,13 +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
@@ -4120,7 +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);
--
1.7.11.7
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