From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 14/23] userfaultfd: wake pending userfaults
Date: Thu, 22 Oct 2015 15:38:24 +0200
Message-ID: <20151022133824.GR17308@twins.programming.kicks-ass.net>
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, qemu-devel@nongnu.org, kvm@vger.kernel.org, linux-api@vger.kernel.org, Pavel Emelyanov <xemul@parallels.com>, Sanidhya Kashyap <sanidhya.gatech@gmail.com>, zhang.zhanghailiang@huawei.com, Linus Torvalds <torvalds@linux-foundation.org>, "Kirill A. Shutemov" <kirill@shutemov.name>, Andres Lagar-Cavilla <andreslc@google.com>, Dave Hansen <dave.hansen@intel.com>, Paolo Bonzini <pbonzini@redhat.com>, Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, Andy Lutomirski <luto@amacapital.net>, Hugh Dickins <hughd@google.com>, Peter Feiner <pfeiner@google.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>
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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

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