Re: [PATCH] Relay CPU Hotplug support

[Oleg Nesterov <oleg@tv-sign.ru>]

On 12/22, Andrew Morton wrote:
>
> On Fri, 22 Dec 2006 16:07:24 +0530
> Gautham R Shenoy <ego@in.ibm.com> wrote:
> 
> > While we are at this per-subsystem cpuhotplug "locking", here's a
> > proposal that might put an end to the workqueue deadlock woes.
> 
> Oleg is working on some patches which will permit us to cancel or wait upon
> a particular work_struct, rather than upon all pending work_structs.  

I hope there are completed. I am waiting for the next -mm release to
send a "final" patch, I need too look at set_wq_data/set_wq_data when
workqueue.c will be in sync with Linus's changes.

> This will fix the problem where we accidentlly wait upon some unrelated
> work_struct which takes a lock which is related to one which we already
> hold.
> 
> I hope.  It'll be a bit tricky to implement: if some foreign work_struct is
> running right now, we cannot wait upon it - we must non-blockingly dequeue
> the work_struct which we want to kill before it gets to run.

The previous patch I sent

	[PATCH, RFC rc1-mm1] implement flush_work()
	http://marc.theaimsgroup.com/?l=linux-kernel&m=116647310413104

has a race.

	 +static void wait_on_work(struct cpu_workqueue_struct *cwq,
	 +                               struct work_struct *work)
	 +{
	 +       struct wq_barrier barr;
	 +       int running = 0;
	 +
	 +       spin_lock_irq(&cwq->lock);
	 +       if (get_wq_data(work) == cwq) {
	 +               list_del_init(&work->entry);
	 +               work_release(work);
	 +       }

If that work is pending on CPU 1 it, and this CPU goes down, it may be
moved to CPU 0 after flush_work() already checked CPU 0.

I think we can do this:

	static void wait_on_work(struct cpu_workqueue_struct *cwq,
					struct work_struct *work)
	{
		struct wq_barrier barr;
		int running = 0;

		spin_lock_irq(&cwq->lock);
		if (unlikely(cwq->current_work == work)) {
			init_wq_barrier(&barr);
			insert_work(cwq, &barr.work, 0);
			running = 1;
		}
		spin_unlock_irq(&cwq->lock);

		if (unlikely(running)) {
			mutex_unlock(&workqueue_mutex);
			wait_for_completion(&barr.done);
			mutex_lock(&workqueue_mutex);
		}
	}

	void flush_work(struct workqueue_struct *wq, struct work_struct *work)
	{
		struct cpu_workqueue_struct *cwq;

		cwq = get_wq_data(work);
		if (!cwq)
			return;

		spin_lock_irq(&cwq->lock);
		list_del_init(&work->entry);
		work_release(work);
		spin_unlock_irq(&cwq->lock);

		mutex_lock(&workqueue_mutex);
		if (is_single_threaded(wq)) {
			/* Always use first cpu's area. */
			wait_on_work(per_cpu_ptr(wq->cpu_wq, singlethread_cpu), work);
		} else {
			int cpu;

			for_each_online_cpu(cpu)
				wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
		}
		mutex_unlock(&workqueue_mutex);
	}

Do you see any problems? When wait_on_work() unlocks workqueue_mutex (or
whatever we choose to protect against CPU hotplug), CPU may go away. But
in that case take_over_work() will move a barrier we queued to another
CPU, it will be fired sometime, and wait_on_work() will be woken.

Actually, we are doing cleanup_workqueue_thread()->kthread_stop() before
take_over_work(), so cwq->thread should complete its ->worklist (and thus
the barrier), because currently we don't check kthread_should_stop() in
run_workqueue(). But even if we did, everything looks safe to me.

Oleg.