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

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Message-ID: <23456.1206387289@vena.lwn.net>
Date:	Mon, 24 Mar 2008 13:34:49 -0600
From:	corbet@....net (Jonathan Corbet)
To:	Alan Stern <stern@...land.harvard.edu>
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>,
	akpm@...ux-foundation.org
Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c 

Alan Stern <stern@...land.harvard.edu> wrote:

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

My fault (again).  Obviously it *looked* like something people could use
to me...  

How about the following patch as a short-term penance to keep others
from making the same mistake?

jon

--

Discourage people from using in_atomic()

Signed-off-by: Jonathan Corbet <corbet@....net>

diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 4982998..3d196cb 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -72,6 +72,11 @@
 #define in_softirq()		(softirq_count())
 #define in_interrupt()		(irq_count())
 
+/*
+ * Are we running in atomic context?  WARNING: this macro cannot
+ * always detect atomic context and should not be used to determine
+ * whether sleeping is possible.  Do not use it in driver code.
+ */
 #define in_atomic()		((preempt_count() & ~PREEMPT_ACTIVE) != 0)
 
 #ifdef CONFIG_PREEMPT

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