netdev - Re: use of preempt_count instead of in

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Message-ID: <Pine.LNX.4.44L0.0803202126240.11234-100000@netrider.rowland.org>
Date:	Thu, 20 Mar 2008 21:31:44 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	David Brownell <david-b@...bell.net>,
	Richard Purdie <rpurdie@...ys.net>,
	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	<netdev@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	<linux-usb@...r.kernel.org>, <linux-wireless@...r.kernel.org>,
	<video4linux-list@...hat.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	<lm-sensors@...sensors.org>
Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c

On Thu, 20 Mar 2008, Andrew Morton wrote:

> On Thu, 20 Mar 2008 21:36:04 -0300 Henrique de Moraes Holschuh <hmh@....eng.br> wrote:
> 
> > Well, so far so good for LEDs, but what about the other users of in_atomic
> > that apparently should not be doing it either?
> 
> Ho hum.  Lots of cc's added.

...

> The usual pattern for most of the above is
> 
> 	if (!in_atomic())
> 		do_something_which_might_sleep();
> 
> problem is, in_atomic() returns false inside spinlock on non-preptible
> kernels.  So if anyone calls those functions inside spinlock they will
> incorrectly schedule and another task can then come in and try take the
> already-held lock.
> 
> Now, it happens that in_atomic() returns true on non-preemtible kernels
> when running in interrupt or softirq context.  But if the above code really
> is using in_atomic() to detect am-i-called-from-interrupt and NOT
> am-i-called-from-inside-spinlock, they should be using in_irq(),
> in_softirq() or in_interrupt().

Presumably most of these places are actually trying to detect 
am-i-allowed-to-sleep.  Isn't that what in_atomic() is supposed to do?  
Why doesn't it do that in non-preemptible kernels?

For that matter, isn't it also the sort of thing that might_sleep() is 
supposed to check?  But looking at the definitions in 
include/linux/kernel.h, it appears that might_sleep() does nothing at 
all when neither CONFIG_PREEMPT_VOLUNTARY nor 
CONFIG_DEBUG_SPINLOCK_SLEEP is set.

Alan Stern

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