On 11/13/2012 07:01 AM, Tejun Heo wrote:
> cgroup->dentry is marked and used as a RCU pointer; however, it isn't
> one - the final dentry put doesn't go through call_rcu(). cgroup and
> dentry share the same RCU freeing rule via synchronize_rcu() in
> cgroup_diput() (kfree_rcu() used on cgrp is unnecessary). If cgrp is
> accessible under RCU read lock, so is its dentry and dereferencing
> cgrp->dentry doesn't need any further RCU protection or annotation.
>
> While not being accurate, before the previous patch, the RCU accessors
> served a purpose as memory barriers - cgroup->dentry used to be
> assigned after the cgroup was made visible to cgroup_path(), so the
> assignment and dereferencing in cgroup_path() needed the memory
> barrier pair. Now that list_add_tail_rcu() happens after
> cgroup->dentry is assigned, this no longer is necessary.
>
> Remove the now unnecessary and misleading RCU annotations from
> cgroup->dentry. To make up for the removal of rcu_dereference_check()
> in cgroup_path(), add an explicit rcu_lockdep_assert(), which asserts
> the dereference rule of @cgrp, not cgrp->dentry.
>
Will memcontrol.c need similar amendments?
The code that lives in -mm and includes kmemcg includes the following
excerpt:
rcu_read_lock();
dentry = rcu_dereference(memcg->css.cgroup->dentry);
rcu_read_unlock();
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