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Re: [PATCH v5 04/20] kthread: Add drain_kthread_worker()

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  Re: [PATCH v5 04/20] kthread: Add drain_kthread_worker() Peter Zijlstra <peterz@infradead.org> - 2016-02-25 13:40 +0100
    Re: [PATCH v5 04/20] kthread: Add drain_kthread_worker() Petr Mladek <pmladek@suse.com> - 2016-02-26 16:30 +0100
      Re: [PATCH v5 04/20] kthread: Add drain_kthread_worker() Peter Zijlstra <peterz@infradead.org> - 2016-02-27 16:20 +0100

#1343144 — Re: [PATCH v5 04/20] kthread: Add drain_kthread_worker()

On Mon, Feb 22, 2016 at 03:56:54PM +0100, Petr Mladek wrote:
> +/**
> + * drain_kthread_worker - drain a kthread worker
> + * @worker: worker to be drained
> + *
> + * Wait until there is no work queued for the given kthread worker.
> + * @worker is flushed repeatedly until it becomes empty.  The number
> + * of flushing is determined by the depth of chaining and should
> + * be relatively short.  Whine if it takes too long.
> + *
> + * The caller is responsible for blocking all users of this kthread
> + * worker from queuing new works. Also it is responsible for blocking
> + * the already queued works from an infinite re-queuing!
> + */
> +void drain_kthread_worker(struct kthread_worker *worker)
> +{
> +	int flush_cnt = 0;
> +
> +	spin_lock_irq(&worker->lock);

Would it not make sense to set a flag here that inhibits (or warns)
queueing new work?

Otherwise this can, as you point out, last forever.

And I think its a logic fail if you both want to drain it and keeping
adding new work.

> +	while (!list_empty(&worker->work_list)) {
> +		spin_unlock_irq(&worker->lock);
> +
> +		flush_kthread_worker(worker);
> +		WARN_ONCE(flush_cnt++ > 10,
> +			  "kthread worker %s: drain_kthread_worker() isn't complete after %u tries\n",
> +			  worker->task->comm, flush_cnt);
> +
> +		spin_lock_irq(&worker->lock);
> +	}
> +
> +	spin_unlock_irq(&worker->lock);
> +}
> +EXPORT_SYMBOL(drain_kthread_worker);
> -- 
> 1.8.5.6
> 

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#1344334

On Thu 2016-02-25 13:35:51, Peter Zijlstra wrote:
> On Mon, Feb 22, 2016 at 03:56:54PM +0100, Petr Mladek wrote:
> > +/**
> > + * drain_kthread_worker - drain a kthread worker
> > + * @worker: worker to be drained
> > + *
> > + * Wait until there is no work queued for the given kthread worker.
> > + * @worker is flushed repeatedly until it becomes empty.  The number
> > + * of flushing is determined by the depth of chaining and should
> > + * be relatively short.  Whine if it takes too long.
> > + *
> > + * The caller is responsible for blocking all users of this kthread
> > + * worker from queuing new works. Also it is responsible for blocking
> > + * the already queued works from an infinite re-queuing!
> > + */
> > +void drain_kthread_worker(struct kthread_worker *worker)
> > +{
> > +	int flush_cnt = 0;
> > +
> > +	spin_lock_irq(&worker->lock);
> 
> Would it not make sense to set a flag here that inhibits (or warns)
> queueing new work?
> 
> Otherwise this can, as you point out, last forever.
> 
> And I think its a logic fail if you both want to drain it and keeping
> adding new work.

We must allow self-queuing because it might be needed to finish
the processing. We would need to detect it. Tejun suggested
to avoid this and make the code simple.

I do not have a strong opinion here. On one hand, such a check might
help with debugging. On the other hand, workqueues have happily lived
without it for years.

Thanks a lot for review,
Petr

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#1345032

On Fri, Feb 26, 2016 at 04:23:09PM +0100, Petr Mladek wrote:
> I do not have a strong opinion here. On one hand, such a check might
> help with debugging. On the other hand, workqueues have happily lived
> without it for years.

TJ and me have a different view on these things. I'm always for the
strictest possible semantics with strong validation. TJ always worries a
lot about existing users.

Luckily, you don't have users yet :-)

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