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

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Message-ID: <20150728171822.GA5322@mtj.duckdns.org>
Date:	Tue, 28 Jul 2015 13:18:22 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Petr Mladek <pmladek@...e.com>
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()

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.

> +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: ...").

Thanks.

-- 
tejun
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