linux-kernel - Re: [PATCH] jump label: introduce static

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Date:	Wed, 23 Feb 2011 10:48:29 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	hpa@...or.com, mathieu.desnoyers@...ymtl.ca, rostedt@...dmis.org,
	mingo@...e.hu, tglx@...utronix.de, andi@...stfloor.org,
	roland@...hat.com, rth@...hat.com, masami.hiramatsu.pt@...achi.com,
	fweisbec@...il.com, avi@...hat.com, davem@...emloft.net,
	sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] jump label: introduce static_branch() interface

On Wed, Feb 23, 2011 at 11:11:25AM +0100, Peter Zijlstra wrote:
> On Tue, 2011-02-22 at 17:28 -0500, Jason Baron wrote:
> > @@ -33,7 +28,11 @@ struct _ddebug {
> >  #define _DPRINTK_FLAGS_PRINT   (1<<0)  /* printk() a message using
> > the format */
> >  #define _DPRINTK_FLAGS_DEFAULT 0
> >         unsigned int flags:8;
> > -       char enabled;
> > +#ifdef HAVE_JUMP_LABEL
> > +       struct jump_label_key enabled;
> > +#else
> > +       int enabled;
> > +#endif
> >  } __attribute__((aligned(8)));
> >   
> 
> Also, didn't you promise Mathieu a comment explaining why dynamic debug
> does funny things like this? :-) I mean we all know its because of the
> cyclic header stuff involving atomic etc.. but I'm also quite sure we'll
> all have forgotten about that in a few weeks time.

ok, how about something along these linnes:

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 7e51b6f..5cc4771 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -1,6 +1,14 @@
 #ifndef _DYNAMIC_DEBUG_H
 #define _DYNAMIC_DEBUG_H
 
+/* 
+ * When jump labels are disabled, they make use of atomic_read(). Since
+ * dynamic_debug.h gets included by kernel.h, the inclusion of jump_label.h by
+ * dynamic_debug.h and thus atomic.h (to get atomic_read()), would cause a
+ * circular dependency since atomic.h requires kernel.h. Thus, for dynamic debug
+ * we are explicitly including jump_label.h when jump labels are configured, and
+ * making use of a regular reads (non-atomic), when jump labels are disabled.
+ */  
 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
 #include <linux/jump_label.h>
 #endif
--
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