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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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