Re: [PATCH 2/2 v2] cgroup: Drop task_lock(parent) on cgroup_fork()

Li Zefan
2011-12-21T02:14:22Z

Frederic Weisbecker wrote:
> We don't need to hold the parent task_lock() on the
> parent in cgroup_fork() because we are already synchronized
> against the two places that may change the parent css_set
> concurrently:
>
> - cgroup_exit(), but the parent obviously can't exit concurrently
> - cgroup migration: we are synchronized against threadgroup_lock()
>
> So we can safely remove the task_lock() there.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: Containers <containers@lists.linux-foundation.org>
> Cc: Cgroups <cgroups@vger.kernel.org>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul Menage <paul@paulmenage.org>
> Cc: Mandeep Singh Baines <msb@chromium.org>
> ---
> kernel/cgroup.c | 10 +++++++---
> 1 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 24f6d6f..1999f60 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4556,7 +4556,7 @@ static const struct file_operations proc_cgroupstats_operations = {
> *
> * A pointer to the shared css_set was automatically copied in
> * fork.c by dup_task_struct(). However, we ignore that copy, since
> - * it was not made under the protection of RCU or cgroup_mutex, so
> + * it was not made under the protection of threadgroup_change_begin(), so

I think the original comment still stands, but now threadgroup_change_begin()
can also protect the cgroup pointer from becoming invalid.

> * might no longer be a valid cgroup pointer. cgroup_attach_task() might
> * have already changed current->cgroups, allowing the previously
> * referenced cgroup group to be removed and freed.
> @@ -4566,10 +4566,14 @@ static const struct file_operations proc_cgroupstats_operations = {
> */
> void cgroup_fork(struct task_struct *child)
> {
> - task_lock(current);
> + /*
> + * We don't need to task_lock() current because current->cgroups
> + * can't be changed concurrently here. The parent obviously hasn't
> + * exited and called cgroup_exit(), and we are synchronized against
> + * cgroup migration through threadgroup_change_begin().
> + */
> child->cgroups = current->cgroups;
> get_css_set(child->cgroups);
> - task_unlock(current);
> INIT_LIST_HEAD(&child->cg_list);
> }
>