linux-kernel - Re: [PATCH v5 04/20] kthread: Add drain_kthread

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Date:	Fri, 26 Feb 2016 16:23:09 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	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>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 04/20] kthread: Add drain_kthread_worker()

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