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Date: Thu, 22 Oct 2015 15:38:24 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH 14/23] userfaultfd: wake pending userfaults
Message-ID: <20151022133824.GR17308@twins.programming.kicks-ass.net>
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On Thu, Oct 22, 2015 at 03:20:15PM +0200, Andrea Arcangeli wrote:

> If schedule spontaneously wakes up a task in TASK_KILLABLE state that
> would be a bug in the scheduler in my view. Luckily there doesn't seem
> to be such a bug, or at least we never experienced it.

Well, there will be a wakeup, just not the one you were hoping for.

We have code that does:

	@cond = true;
	get_task_struct(p);
	queue(p)

				/* random wait somewhere */
				for (;;) {
					prepare_to_wait();
					if (@cond)
					  break;

				...

				handle_userfault()
				  ...
				  schedule();
	...

	dequeue(p)
	wake_up_process(p) ---> wakeup without userfault wakeup


These races are (extremely) rare, but they do exist. Therefore one must
never assume schedule() will not spuriously wake because of these
things.

Also, see:

lkml.kernel.org/r/CA+55aFwHkOo+YGWKYROmce1-H_uG3KfEUmCkJUerTj=ojY2H6Q@mail.gmail.com