linux-kernel - Re: [RFC][PATCH 1/7] wait: Provide infrastructure to deal with nested blocking

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Message-ID: <20140804183500.GA20127@redhat.com>
Date:	Mon, 4 Aug 2014 20:35:00 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, torvalds@...ux-foundation.org,
	tglx@...utronix.de, ilya.dryomov@...tank.com,
	umgwanakikbuti@...il.com, linux-kernel@...r.kernel.org,
	Eric Paris <eparis@...isplace.org>,
	John McCutchan <john@...nmccutchan.com>,
	Robert Love <rlove@...ve.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [RFC][PATCH 1/7] wait: Provide infrastructure to deal with
	nested blocking

On 08/04, Peter Zijlstra wrote:
>
> I just noticed that poll_schedule_timeout() uses set_mb() for clearing
> its triggered variable. But I'm not entirely sure I see why..

At least we need to ensure that this "pwq->triggered = 0" can't be reordered
after the next ->poll() returns 0. In this case we should sleep unless pollwake()
was called and it set "->triggered = 1". So without this mb() we can miss an
event.

But I can hardly understand the "data written before wake up is always visible
after wake up" part... Probably this means that if we didn't actually sleep
because pwq->triggered == 1 we need to ensure that the next ->poll(file) should
see all changes which were the reason for wakeup.

Oleg.

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