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

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Message-ID: <20160225123551.GG6357@twins.programming.kicks-ass.net>
Date:	Thu, 25 Feb 2016 13:35:51 +0100
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 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
>