linux-kernel - Re: use of preempt_count instead of in

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Date:	Fri, 21 Mar 2008 14:05:59 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Jean Delvare <khali@...ux-fr.org>, Michael Buesch <mb@...sch.de>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	David Brownell <david-b@...bell.net>,
	Richard Purdie <rpurdie@...ys.net>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c

On Fri, 21 Mar 2008, Andrew Morton wrote:

> On Fri, 21 Mar 2008 10:53:11 +0100 Jean Delvare <khali@...ux-fr.org> wrote:
> 
> > On Thu, 20 Mar 2008 20:17:23 -0700, Andrew Morton wrote:
> > > in_atomic() is for core kernel use only.  (...)
> > 
> > Then why is it made available to drivers through <linux/hardirq.h>?
> 
> Because we suck.
> 
> > If
> > it's such a dangerous macro to call from drivers, it shouldn't be made
> > available, or at the very least there should be a big fat warning in
> > <linux/hardirq.h> that drivers aren't supposed to use it. This would
> > have avoided the 23 uses cases in drivers we have right now.
> 
> True.

There's also a section about in_atomic() in the Linux Device Drivers 
(3rd ed.) book which may have contributed to the confusion.  On p. 198:

	A function related to in_interrupt() is in_atomic().  Its 
	return value is nonzero whenever scheduling is not allowed;
	this includes hardware and software interrupt contexts as well
	as any time when a spinlock is held.  In the latter case, 
	current may be valid, but access to user space is forbidden, 
	since it can cause scheduling to happen.  Whenever you are
	using in_interrupt(), you should really consider whether 
	in_atomic() is what you actually mean.  Both functions are
	declared in <asm/hardirq.h>.

Alan Stern

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