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

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Date:	Mon, 24 Mar 2008 13:53:17 -0600
From:	corbet@....net (Jonathan Corbet)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	stern@...land.harvard.edu, khali@...ux-fr.org, mb@...sch.de,
	hmh@....eng.br, david-b@...bell.net, rpurdie@...ys.net,
	linux-kernel@...r.kernel.org, mingo@...e.hu, geert@...ux-m68k.org
Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c 

Andrew Morton <akpm@...ux-foundation.org> wrote:

> It'd be better if the comment were to describe _why_ in_atomic() is
> unreliable.  ie: "does not account for held spinlocks on non-preemptible
> kernels".

But then...why would anybody have a reason to read the upcoming LWN
article on the subject?  

OK, how's this?

jon

--

Discourage people from inappropriately using in_atomic()

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

diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 4982998..63a7782 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -72,6 +72,13 @@
 #define in_softirq()		(softirq_count())
 #define in_interrupt()		(irq_count())
 
+/*
+ * Are we running in atomic context?  WARNING: this macro cannot
+ * always detect atomic context; in particular, it cannot know about
+ * held spinlocks in non-preemptible kernels.  Thus it should not be
+ * used in the general case to determine whether sleeping is possible.
+ * Do not use in_atomic() in driver code.
+ */
 #define in_atomic()		((preempt_count() & ~PREEMPT_ACTIVE) != 0)
 
 #ifdef CONFIG_PREEMPT
--
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