linux-kernel - Re: [PATCH 1/7] sched,wait: Fix a kthread race with wait

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Message-ID: <20141031221340.GA28563@redhat.com>
Date:	Fri, 31 Oct 2014 23:13:40 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	peter@...leysoftware.com, rjw@...ysocki.net,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	eparis@...hat.com, umgwanakikbuti@...il.com, marcel@...tmann.org,
	ebiederm@...ssion.com, davem@...emloft.net, fengguang.wu@...el.com
Subject: Re: [PATCH 1/7] sched,wait: Fix a kthread race with wait_woken()

On 10/31, Peter Zijlstra wrote:
>
> There is a race between kthread_stop() and the new wait_woken() that
> can result in a lack of progress.

Likewise, the user of wait_woken() can miss any other event which is
not associated with wq we are going to sleep on. Please see below.

> +static inline bool is_kthread_should_stop(void)
> +{
> +	return (current->flags & PF_KTHREAD) && kthread_should_stop();
> +}
>
>  /*
>   * DEFINE_WAIT_FUNC(wait, woken_wake_func);
> @@ -326,7 +331,7 @@ long wait_woken(wait_queue_t *wait, unsi
>  	 * woken_wake_function() such that if we observe WQ_FLAG_WOKEN we must
>  	 * also observe all state before the wakeup.
>  	 */
> -	if (!(wait->flags & WQ_FLAG_WOKEN))
> +	if (!(wait->flags & WQ_FLAG_WOKEN) && !is_kthread_should_stop())
>  		timeout = schedule_timeout(timeout);

Well yes, this is more straightforward than other hacks we discussed before.
But see above, this doesn't look flexible enough.

And. This assumes that the user must also check kthread_should_stop(),
otherwise the waiting loop becomes a busy-wait loop.

So I won't argue, but I still think it would be better to allow the user to
do set_task_state() by hand if it needs to check the additional conditions.

Oleg.

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