linux-kernel - Re: [RFC PATCH 03/14] kthread: Add drain_kthread

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Date:	Wed, 29 Jul 2015 12:04:57 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	Vlastimil Babka <vbabka@...e.cz>,
	live-patching@...r.kernel.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 03/14] kthread: Add drain_kthread_worker()

On Tue 2015-07-28 13:18:22, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jul 28, 2015 at 04:39:20PM +0200, Petr Mladek wrote:
> > +/*
> > + * Test whether @work is being queued from another work
> > + * executing on the same kthread.
> > + */
> > +static bool is_chained_work(struct kthread_worker *worker)
> > +{
> > +	struct kthread_worker *current_worker;
> > +
> > +	current_worker = current_kthread_worker();
> > +	/*
> > +	 * Return %true if I'm a kthread worker executing a work item on
> > +	 * the given @worker.
> > +	 */
> > +	return current_worker && current_worker == worker;
> > +}
> 
> I'm not sure full-on chained work detection is necessary here.
> kthread worker's usages tend to be significantly simpler and draining
> is only gonna be used for destruction.

I think that it might be useful to detect bugs when someone
depends on the worker when it is being destroyed. For example,
I tried to convert "khubd" kthread and there was not easy to
double check that this worked as expected.

I actually think about replacing

    WARN_ON_ONCE(!is_chained_work(worker)))

with

    WARN_ON(!is_chained_work(worker)))

in queue_kthread_work, so that we get the warning for all misused
workers.

> > +void drain_kthread_worker(struct kthread_worker *worker)
> > +{
> > +	int flush_cnt = 0;
> > +
> > +	spin_lock_irq(&worker->lock);
> > +	worker->nr_drainers++;
> > +
> > +	while (!list_empty(&worker->work_list)) {
> > +		/*
> > +		 * Unlock, so we could move forward. Note that queuing
> > +		 * is limited by @nr_drainers > 0.
> > +		 */
> > +		spin_unlock_irq(&worker->lock);
> > +
> > +		flush_kthread_worker(worker);
> > +
> > +		if (++flush_cnt == 10 ||
> > +		    (flush_cnt % 100 == 0 && flush_cnt <= 1000))
> > +			pr_warn("kthread worker %s: drain_kthread_worker() isn't complete after %u tries\n",
> > +				worker->task->comm, flush_cnt);
> > +
> > +		spin_lock_irq(&worker->lock);
> > +	}
> 
> I'd just do something like WARN_ONCE(flush_cnt++ > 10, "kthread worker: ...").

This would print the warning only for one broken worker. But I do not
have strong opinion about it.

Best Regards,
Petr
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