Frederic Weisbecker wrote:
> cgroup_post_fork() is protected between threadgroup_change_begin()
> and threadgroup_change_end() against concurrent changes of the
> child's css_set in cgroup_task_migrate(). Also the child can't
> exit and call cgroup_exit() at this stage, this means it's css_set
> can't be changed with init_css_set concurrently.
>
> For these reasons, we don't need to hold task_lock() on the child
> because it's css_set can only remain stable in this place.
>
> Let's remove the lock there.
>
> NOTE: We could do something else: move threadgroup_change_end()
> before cgroup_post_fork() and keep the task_lock() which, combined
> with the css_set_lock, would be enough to synchronize against
> cgroup_task_migrate()'s change on child->cgroup and its cglist.
> Because outside that, the threadgroup lock doesn't appear to be
> needed on cgroup_post_fork().
>
To narrow the scope of the threadgroup lock? I think it's preferable to keep
cgroup_post_fork() inside the lock, to make things simpler and we have
the same lock rule for both cgroup_fork() and cgroup_post_fork().
> Let's debate!
>
> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Containers <containers@xxxxxxxxxxxxxxxxxxxxxxxxxx>
> Cc: Cgroups <cgroups@xxxxxxxxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Paul Menage <paul@xxxxxxxxxxxxxx>
> Cc: Mandeep Singh Baines <msb@xxxxxxxxxxxx>
> ---
> kernel/cgroup.c | 11 ++++++++---
> 1 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 4936d88..d0dbf72 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4622,10 +4622,15 @@ void cgroup_post_fork(struct task_struct *child)
> {
> if (use_task_css_set_links) {
> write_lock(&css_set_lock);
> - task_lock(child);
> - if (list_empty(&child->cg_list))
> + if (list_empty(&child->cg_list)) {
> + /*
> + * It's safe to use child->cgroups without task_lock()
> + * here because we are protected through
> + * threadgroup_change_begin() against concurrent
> + * css_set change in cgroup_task_migrate()
> + */
Also explain why it won't race with cgroup_exit()? You were not quite confident
about that before Oleg's explanation. ;)
> list_add(&child->cg_list, &child->cgroups->tasks);
> - task_unlock(child);
> + }
> write_unlock(&css_set_lock);
> }
> }
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