linux-kernel - Re: [PATCH v3 09/22] kthread: Allow to cancel kthread work

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Date:	Mon, 23 Nov 2015 17:58:23 -0500
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>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 09/22] kthread: Allow to cancel kthread work

Hello,

On Wed, Nov 18, 2015 at 02:25:14PM +0100, Petr Mladek wrote:
> +static int
> +try_to_cancel_kthread_work(struct kthread_work *work,
> +				   spinlock_t *lock,
> +				   unsigned long *flags)
> +{
> +	int ret = 0;
> +
> +	if (work->timer) {
> +		/* Try to cancel the timer if pending. */
> +		if (del_timer(work->timer)) {
> +			ret = 1;
> +			goto out;
> +		}
> +
> +		/* Are we racing with the timer callback? */
> +		if (timer_active(work->timer)) {
> +			/* Bad luck, need to avoid a deadlock. */
> +			spin_unlock_irqrestore(lock, *flags);
> +			del_timer_sync(work->timer);
> +			ret = -EAGAIN;
> +			goto out;
> +		}

As the timer side is already kinda trylocking anyway, can't the cancel
path be made simpler?  Sth like

	lock(worker);
	work->canceling = true;
	del_timer_sync(work->timer);
	unlock(worker);

And the timer can do (ignoring the multiple worker support, do we even
need that?)

	while (!trylock(worker)) {
		if (work->canceling)
			return;
		cpu_relax();
	}
	queue;
	unlock(worker);

Thanks.

-- 
tejun
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