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    SubjectRe: [PATCH UPDATED 03/10] threadgroup: extend threadgroup_lock() to cover exit and exec
    On Mon, Dec 05, 2011 at 10:43:15AM -0800, Tejun Heo wrote:
    > Hello, Frederic.
    >
    > On Fri, Dec 02, 2011 at 05:28:03PM +0100, Frederic Weisbecker wrote:
    > > But I don't think it's very useful to protect against irq_exit_thread(),
    > > what happens there is purely of internal irq interest.
    > >
    > > Then right after, PF_EXITING is set before any interesting change.
    > > Isn't it possible to simply lock this flag setting? IIRC, as soon
    > > as the PF_EXITING flag is set, you ignore the task for attachment.
    >
    > I think that's technically possible but it does introduce another
    > class of tasks - the dying ones. e.g. If a task has PF_EXITING set
    > and the containing process is migrating, we'll have to migrate all
    > tasks but the dying one and cgroup ->exit callbacks can be called on
    > the lonely task after the migration is complete. It's kinda messy and
    > if someone makes a wrong assumption there, the bug is gonna be even
    > more difficult to reproduce / track down than now. Yes, smaller scope
    > locking is nicer but I would like to avoid api weirdities like that.

    I don't understand what you mean. On your patches, you only process tasks
    that don't have PF_EXITING, ie: you don't include these in the flex array
    on cgroup_attach_proc(). So that still applies in my proposal.

    From the exit path we would have:

    exit_signal() {
    lock_threadgroup_change(task);
    task->flags |= PF_EXITING;
    lock_threadgroup(task);
    }

    exit all the rest: mm, etc...

    Then from cgroup_attach_proc():

    lock_threadgroup(task);
    for_each_thread(task) {
    if (!(task->flags & PF_EXITING))
    include in flex array
    }

    Am I forgetting something?


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