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SubjectRe: [PATCH 04/10] cgroup: always lock threadgroup during migration
On Tue, Nov 01, 2011 at 04:46:27PM -0700, Tejun Heo wrote:
> Update cgroup to take advantage of the fack that threadgroup_lock()
> guarantees stable threadgroup.
>
> * Lock threadgroup even if the target is a single task. This
> guarantees that when the target tasks stay stable during migration
> regardless of the target type.
>
> * Remove PF_EXITING early exit optimization from attach_task_by_pid()
> and check it in cgroup_task_migrate() instead. The optimization was
> for rather cold path to begin with and PF_EXITING state can be
> trusted throughout migration by checking it after locking
> threadgroup.
>
> * Don't add PF_EXITING tasks to target task array in
> cgroup_attach_proc(). This ensures that task migration is performed
> only for live tasks.
>
> * Remove -ESRCH failure path from cgroup_task_migrate(). With the
> above changes, it's guaranteed to be called only for live tasks.
>
> After the changes, only live tasks are migrated and they're guaranteed
> to stay alive until migration is complete. This removes problems
> caused by exec and exit racing against cgroup migration including
> symmetry among cgroup attach methods and different cgroup methods
> racing each other.
>
> v2: Oleg pointed out that one more PF_EXITING check can be removed
> from cgroup_attach_proc(). Removed.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul Menage <paul@paulmenage.org>
> Cc: Li Zefan <lizf@cn.fujitsu.com>

Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>

Just a little thing:

> ---
> kernel/cgroup.c | 51 +++++++++++++++++++++++----------------------------
> 1 files changed, 23 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index f0e099f..83e10f9 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1762,7 +1762,7 @@ EXPORT_SYMBOL_GPL(cgroup_path);
> *
> * 'guarantee' is set if the caller promises that a new css_set for the task
> * will already exist. If not set, this function might sleep, and can fail with
> - * -ENOMEM. Otherwise, it can only fail with -ESRCH.
> + * -ENOMEM. Must be called with cgroup_mutex and threadgroup locked.
> */
> static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
> struct task_struct *tsk, bool guarantee)
> @@ -1800,13 +1800,9 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
> }
> put_css_set(oldcg);
>
> - /* if PF_EXITING is set, the tsk->cgroups pointer is no longer safe. */
> + /* @tsk can't exit as its threadgroup is locked */
> task_lock(tsk);
> - if (tsk->flags & PF_EXITING) {
> - task_unlock(tsk);
> - put_css_set(newcg);
> - return -ESRCH;
> - }
> + WARN_ON_ONCE(tsk->flags & PF_EXITING);

I have the feeling the task_lock is now useless given that we are synchronized
against cgroup_exit() with the threadgroup_lock.

It's probably also useless in cgroup_exit(). But that's something for a further
patch...

Thanks.


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