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    SubjectRe: [PATCHSET cgroup/for-3.8] cpuset: decouple cpuset locking from cgroup core
    On 11/30/2012 01:24 PM, Michal Hocko wrote:
    > On Fri 30-11-12 13:00:36, Glauber Costa wrote:
    >> On 11/30/2012 07:21 AM, Kamezawa Hiroyuki wrote:
    >>> (2012/11/29 6:34), Tejun Heo wrote:
    >>>> Hello, guys.
    >>>>
    >>>> Depending on cgroup core locking - cgroup_mutex - is messy and makes
    >>>> cgroup prone to locking dependency problems. The current code already
    >>>> has lock dependency loop - memcg nests get_online_cpus() inside
    >>>> cgroup_mutex. cpuset the other way around.
    >>>>
    >>>> Regardless of the locking details, whatever is protecting cgroup has
    >>>> inherently to be something outer to most other locking constructs.
    >>>> cgroup calls into a lot of major subsystems which in turn have to
    >>>> perform subsystem-specific locking. Trying to nest cgroup
    >>>> synchronization inside other locks isn't something which can work
    >>>> well.
    >>>>
    >>>> cgroup now has enough API to allow subsystems to implement their own
    >>>> locking and cgroup_mutex is scheduled to be made private to cgroup
    >>>> core. This patchset makes cpuset implement its own locking instead of
    >>>> relying on cgroup_mutex.
    >>>>
    >>>> cpuset is rather nasty in this respect. Some of it seems to have come
    >>>> from the implementation history - cgroup core grew out of cpuset - but
    >>>> big part stems from cpuset's need to migrate tasks to an ancestor
    >>>> cgroup when an hotunplug event makes a cpuset empty (w/o any cpu or
    >>>> memory).
    >>>>
    >>>> This patchset decouples cpuset locking from cgroup_mutex. After the
    >>>> patchset, cpuset uses cpuset-specific cpuset_mutex instead of
    >>>> cgroup_mutex. This also removes the lockdep warning triggered during
    >>>> cpu offlining (see 0009).
    >>>>
    >>>> Note that this leaves memcg as the only external user of cgroup_mutex.
    >>>> Michal, Kame, can you guys please convert memcg to use its own locking
    >>>> too?
    >>>>
    >>>
    >>> Hmm. let me see....at quick glance cgroup_lock() is used at
    >>> hierarchy policy change
    >>> kmem_limit
    >>> migration policy change
    >>> swapiness change
    >>> oom control
    >>>
    >>> Because all aboves takes care of changes in hierarchy,
    >>> Having a new memcg's mutex in ->create() may be a way.
    >>>
    >>> Ah, hm, Costa is mentioning task-attach. is the task-attach problem in memcg ?
    >>>
    >>
    >> We disallow the kmem limit to be set if a task already exists in the
    >> cgroup. So we can't allow a new task to attach if we are setting the limit.
    >
    > This is racy without additional locking, isn't it?
    >
    Apparently, the way Tejun fixed this for cpuset was by using the
    "attach_in_progress" indicator, that IIUC, is flipped up in
    ->can_attach, and down in ->attach.

    A similar scheme would work for us.

    And we should also be using a similar scheme for cgroup creation:
    the css is not really connected to the parent until after
    memcg_alloc_css. So if we use the memcg iterator to figure out if
    children exist, we may get a race where we believe no children exist,
    but one appear right after.


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