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    SubjectRe: [PATCH 7/8] Clean up workqueue.c with respect to the freezer based cpu-hotplug
    On Tue, Apr 03, 2007 at 07:03:36PM +0400, Oleg Nesterov wrote:
    > I think it would be nice to do. I believe we can cleanup ksoftirqd()
    > and migration_thread() as well (kill wait_to_die: loop). Probably it

    I doubt whether we can kill it in migration_thread, since that is
    another thread which is unfrozen for hotplug (stop_machine relies on its
    services while rest of the world is frozen).

    > is better to introduce a new helper for that, kthread_thaw_stop() or
    > something.

    Will think of that.


    > > Why?
    >
    > What if is_single_threaded(wq) == true? In that case we should call
    > flush_cpu_workqueue(cpu) only if cpu == singlethread_cpu, otherwise
    > this is unneeded and wrong, because per_cpu_ptr(wq->cpu_wq, cpu) was
    > not initialized.

    Ah yes ..

    > > kthread_stop(p)
    > > {
    > > int old_exempt_flags;
    > >
    > > task_lock(p);
    > > old_exempt_flags = p->flags;
    > > p->flags |= PFE_ALL; /* Exempt 'p' from being frozen? */
    >
    > I agree, we should mark this thread as non-freezable, but we can't modify
    > p->flags, this is racy. "current" owns its ->flags and it is not atomic.
    > Note that thaw_process() checks frozen(p) when it clears PF_FROZEN.

    I suspected that we cannot modify p->flags just like that. How abt
    moving freezer exemption bits to a separate field, which is protected by
    task_lock?

    > Actually, we should do this before destroy_workqueue() calls flush_workqueue().
    > Otherwise flush_cpu_workqueue() can hang forever in a similar manner.

    Yep. I guess these are a class of freezer deadlocks very similar to vfork
    parent waiting on child case. I get a feeling these should become common
    outside of kthread too (A waits on B for something, B gets frozen, which
    means A won't freeze causing freezer to fail). Can freezer detect this
    dependency somehow and thaw B automatically? Probably not that easy ..

    > Needs more thinking, I guess.

    [snip]

    > No, no, workqueue_mutex can't help. Just for example: CPU_UP_PREPARE completes
    > and drops workqueue_mutex. __create_workqueue(wq) doesn't see the new cpu, it
    > is not on cpu_online_map, so it doesn't create cwq->thread. CPU_ONLINE oopses.

    Ok ..sure.

    --
    Regards,
    vatsa
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