linux-kernel - Re: [PATCH 2/7] wait: Reimplement wait_event

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Message-ID: <20141031213802.GA25881@redhat.com>
Date:	Fri, 31 Oct 2014 22:38:02 +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 2/7] wait: Reimplement wait_event_freezable()

On 10/31, Peter Zijlstra wrote:
>
> Provide better implementations of wait_event_freezable() APIs.
>
> The problem is with freezer_do_not_count(), it hides the thread from
> the freezer, even though this thread might not actually freeze/sleep
> at all.

I agree, wait_event_freezable() is awful. But could you clarify "at all" ?

Sure, the task can be preempted right after it sets, it can do a lot
of things before it calls schedule(), it can be woken after that and
it can run again and do something else before freezer_count() calls
try_to_freeze(), etc.

Is this what you meant?

> +#define __wait_event_freezable(wq, condition)				\
> +	(void)___wait_event(wq, condition, TASK_INTERRUPTIBLE, 0, 0,	\
> +			    schedule(); try_to_freeze())

I don't think this can work. wait_event_freezable() should be used by
kernel threads and thus we can't rely on TIF_SIGPENDING, freeze_task()
simply does wake_up_state(TASK_INTERRUPTIBLE) in this case.

Just for example, suppose that try_to_freeze_tasks() calls freeze_task()
before this kthread sets current->state = TASK_INTERRUPTIBLE. In this
case __wait_event_freezable()->schedule() will happily sleep and
try_to_freeze_tasks() will fail.

That is why I tried to suggest cmd == freezable_schedule(). Still not
good, but at least this narrows the window and (perhaps) we can improve
this freezable_schedule() later.

But on a second thought... Probably cmd => try_to_freeze(); schedule();
should work. Or just

	#define __wait_event_freezable(wq, condition)	\
		__wait_event_interruptible(wq, ({ try_to_freeze(); (condition); }))

which looks simpler.

Oleg.

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