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

[Li Zefan <lizf@cn.fujitsu.com>]

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);
>  }
>