linux-kernel - Re: [PATCH/RFC] wait_for_completion

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Message-ID: <20131119082548.GD10022@twins.programming.kicks-ass.net>
Date:	Tue, 19 Nov 2013 09:25:48 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	NeilBrown <neilb@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	"Dr. H. Nikolaus Schaller" <hns@...delico.com>,
	Marek Belisko <marek@...delico.com>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH/RFC] wait_for_completion_timeout() considered harmful.

On Tue, Nov 19, 2013 at 10:44:38AM +1100, NeilBrown wrote:
> We have loops that have
>     timeout = schedule_timeout(timeout)
> in the middle and if we change the semantics of schedule_timeout() to round
> up, those loops could wait quite a bit longer than expected.

Depends on what you expect; most of these functions have documentation
that says they will sleep at least timeout amount of time.

schedule_timeout()'s version looks like:

 * Make the current task sleep until @timeout jiffies have
 * elapsed.

Clearly it doesn't do that currently, so adding 1 will actually make it
do what it says on the tin.
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