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DateTue, 25 Mar 2008 11:39:42 +0100
FromJean Delvare <>
SubjectRe: use of preempt_count instead of in_atomic() at leds-gpio.c
Digg This
On Tue, 25 Mar 2008 01:52:58 -0700, Junio C Hamano wrote:
corbet@lwn.net (Jonathan Corbet) writes:

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

Is it just me who feels this comment that says "in_atomic() is not a way to tell if we are in atomic reliably and cannot be used for such and such" very reader-unfriendly? Ok, maybe the macro is not reliable and is not meant to be used for the purpose its name seems to suggest (at least to a non-kernel person). An inevitable question is, then what is it good for? What's the right situation to use this macro?

I guess an additional comment "even if this says no, you could still be in atomic, but if this says yes, then you definitely are in atomic and cannot sleep" may help unconfuse a clueless reader like myself.

Andrew explained that in_atomic() could deadlock if called in a condition where it is unreliable (although I did not understand the details). Documenting that a "yes" from in_atomic() can always be trusted, would invite driver authors to still use it, when my understanding is that they still shouldn't.

If drivers shouldn't use in_atomic() at all then I think that the long-term solution is to move its definition out of <linux/hardirq.h>. But of course this means fixing all the drivers that still use it first.
-- 
Jean Delvare


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