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

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Message-Id: <20131118152746.937b2b7971d7a4bba4ef996d@linux-foundation.org>
Date:	Mon, 18 Nov 2013 15:27:46 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	NeilBrown <neilb@...e.de>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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@...nsource.wolfsonmicro.com>
Subject: Re: [PATCH/RFC] wait_for_completion_timeout() considered harmful.

On Sun, 17 Nov 2013 08:06:03 +1100 NeilBrown <neilb@...e.de> wrote:

> It would be reasonable to assume that
> 
>       wait_for_completion_timeout(&wm8350->auxadc_done, msecs_to_jiffies(5));
> 
> would wait at least 5 msecs for the auxadc_done to complete.  But it does not.
> With a HZ of 200 or less, msecs_to_jiffies(5) has value '1', and so this
> will only wait until the next "timer tick", which could happen immediately.
> 
> This can lead to incorrect results - and has done so in out-of-tree patches
> for drivers/misc/bmp085.c which uses a very similar construct to enable interrupt
> based result collection.
> 
> The documentation for several *_timeout* functions claim they will wait for
> "timeout jiffies" to have elapsed where this is not the case.  They will
> actually wait for "timeout" jiffies to have started implying an elapsed time
> between (timeout-1) and (timeout).
> 
> This patch corrects some of this documentation, and adds a collection of
>   wait_for_completion*_msecs()
> interfaces which wait at least the given number of milliseconds for the
> completion (or a signal).

Mutter.  wait_for_x(..., 5ms) should wait for a minimum of 5ms, no matter
what.

So I'd suggest we make that happen, rather than adding some new interfaces?
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