linux-kernel - Re: [PATCH v9 06/12] kthread: Add kthread_drain

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Date:	Wed, 22 Jun 2016 22:54:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Petr Mladek <pmladek@...e.com>
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 v9 06/12] kthread: Add kthread_drain_worker()

On Thu, Jun 16, 2016 at 01:17:25PM +0200, Petr Mladek wrote:
> +/**
> + * kthread_drain_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!

This, I really dislike that. And it makes the kthread_destroy_worker()
from the next patch unnecessarily fragile.

Why not add a kthread_worker::blocked flag somewhere and refuse/WARN
kthread_queue_work() when that is set.

> + */
> +void kthread_drain_worker(struct kthread_worker *worker)
> +{
> +	int flush_cnt = 0;
> +
> +	spin_lock_irq(&worker->lock);
> +
> +	while (!list_empty(&worker->work_list)) {
> +		spin_unlock_irq(&worker->lock);
> +
> +		kthread_flush_worker(worker);
> +		WARN_ONCE(flush_cnt++ > 10,
> +			  "kthread worker %s: kthread_drain_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(kthread_drain_worker);
> -- 
> 1.8.5.6
>