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    SubjectRe: Freezeable workqueues [Was: 2.6.22-rc1: Broken suspend on SMP with tifm]
    On 05/15, Rafael J. Wysocki wrote:
    > On Monday, 14 May 2007 23:48, Oleg Nesterov wrote:
    > >
    > > So, in the long term, should we change this only user, or we think we better fix
    > > freezeable wqs again?
    > Long term, I'd like to have freezable workqueues, so that people don't have to
    > use "raw" kernel threads only because they need some synchronization with
    > hibertnation/suspend. Plus some cases in which workqueues are used by
    > fs-related code make me worry.

    OK, so we should fix them. It would be great to also fix the last known problem
    as well (work->func() vs hotplug callback deadlocks).

    I am a bit afraid of too many yes/no options for the freezer, a couple of naive

    1. Can't we make all wqs freezable? I still can't see the reason to have both
    freezable and not freezable wqs.

    2. Why do we need CPU_TASKS_FROZEN? Can't we change cpu-hotplug to always
    freeze tasks right now, without any additional changes?

    Any subsystem should handle correctly the case when _cpu_down() (say)
    is called with tasks_frozen == 1 anyway. So, why can't we simplify
    things and do

    _cpu_down(int tasks_frozen)

    if (!tasks_frozen)

    right now?

    > [*] The problem is, though, that freezable workqueus have some potential to fail
    > the freezer. Namely, suppose task A calls flush_workqueue() on a freezable
    > workqueue, finds some work items in there, inserts the barrier and waits for
    > completion (TASK_UNINTERRUPTIBLE). In the meantime, TIF_FREEZE is set on
    > the worker thread, which is then woken up and goes to the refrigerator. Thus
    > if A is not NOFREEZE, the freezing of tasks will fail (A must be a kernel
    > thread for this to happen, but still). Worse yet, if A is NOFREEZE, it will be
    > blocked until the worker thread is woken up.

    Yes, this is yet another dependency which freezer can't handle. Probably it is
    better to ignore this problem for now.

    > To avoid this, I think, we may need to redesign the freezer, so that freezable
    > worker threads are frozen after all of the other kernel threads.

    I doubt we can find a very clean way to do this. Besides, what if work->func()
    does flush_workqueue(another_wq) ? How can we decide which wq to freeze first?

    > Additionally,
    > we'd need to make a rule that NOFREEZE kernel threads must not call
    > flush_workqueue() or cancel_work_sync() on freezable workqueues.

    cancel_work_sync() is OK, it can be used safely even if workqueue is frozen.
    flush_workqueue() and destroy_workqueue() are not.


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