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

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Message-Id: <20080318001429.896acf51.akpm@linux-foundation.org>
Date:	Tue, 18 Mar 2008 00:14:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Brownell <david-b@...bell.net>
Cc:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Richard Purdie <rpurdie@...ys.net>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c

On Sun, 16 Mar 2008 11:46:23 -0800 David Brownell <david-b@...bell.net> wrote:

> On Sunday 16 March 2008, Henrique de Moraes Holschuh wrote:
> > Is the use of "if (preempt_count())" to know when to defer led gpio work to
> > a workqueue needed? __Shouldn't "if (in_atomic())" be enough?
> 
> At this point, I don't know of any such reason.
> 
> I remember hunting for the right heuristic, and settling on
> that one for reasons that I can't recall now.  They may even
> be no longer applicable.

Both are incorrect.  When CONFIG_PREEMPT=n we have no support for
determining whether schedule() may be called.  The calling code has to sort
out its stuff on its own.

<greps for preempt_count>

The LEDs code seems to be the sole offender.  print_vma_addr() might be
wrong too, but Ingo did it, and perhaps he knows that all code paths which
call print_vma_addr() from deadlockable contexts have already called
inc_preempt_count().  But is that true for all architectures?

<greps for in_atomic>

omigawd, what have we done, and how can we fix it? :(
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