linux-kernel - [RFC 2/2] printk/bug: Remove cyclic dependency between bug.h and printk.h

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Date:   Tue, 11 Jan 2022 15:30:46 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Chris Down <chris@...isdown.name>,
        Marc Zyngier <maz@...nel.org>,
        Andrew Scull <ascull@...gle.com>,
        Will Deacon <will@...nel.org>, Jason Baron <jbaron@...mai.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: [RFC 2/2] printk/bug: Remove cyclic dependency between bug.h and printk.h

`make headerdep` reports many circular dependencies with the same
pattern:

In file included from linux/bug.h,
                 from linux/jump_label.h:262
                 from linux/dynamic_debug.h:6
                 from linux/printk.h:504
                 from asm-generic/bug.h:22
                 from linux/bug.h:32 <-- here

It does not cause real problem because 'asm-generic/bug.h' uses only
plain printk() and no_printk(). And these two functions are defined
in 'printk.h' before 'dynamic_debug.h' is included.

There is no easy solution because all affected code does some inline
tricks:

  + printk() adds struct pi_entry metadata
  + dynamic_pr_debug() adds struct _ddebug metadata
  + static_branch_likely() adds assembly that realizes the jump
  + BUG() prints __FILE__, __LINE__, __func__ where it is inlined

One solution would be to modify BUG() and pass __FILE__, __LINE__,
__func__ into a helper function implemented in a .c source file.
It will not require including "printk.h" in "bug.h". The drawback
is code complication.

Alternative solution is to include "printk_core.h" and use the raw
_printk(). The drawback is that the string will not be listed in
printk index. But it will afftect only few architectires that do
not define HAVE_ARCH_BUG.

This patch uses the alternative solution because it seems to be
easier to maintain. The BUG() definitions are already complicated
enough thanks to all the ifdefs.

Reported-by: Andy Shevchenko <andy.shevchenko@...il.com>
Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 include/asm-generic/bug.h   |  4 ++--
 include/linux/printk.h      | 11 -----------
 include/linux/printk_core.h | 11 +++++++++++
 3 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index edb0e2a602a8..140afb8bdfe7 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -19,7 +19,7 @@
 
 #ifndef __ASSEMBLY__
 #include <linux/panic.h>
-#include <linux/printk.h>
+#include <linux/printk_core.h>
 
 #ifdef CONFIG_BUG
 
@@ -55,7 +55,7 @@ struct bug_entry {
  */
 #ifndef HAVE_ARCH_BUG
 #define BUG() do { \
-	printk("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__, __func__); \
+	_printk("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__, __func__); \
 	barrier_before_unreachable(); \
 	panic("BUG!"); \
 } while (0)
diff --git a/include/linux/printk.h b/include/linux/printk.h
index c20f55df7fa6..23530b0a2a07 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -123,17 +123,6 @@ struct va_format {
  */
 #define DEPRECATED	"[Deprecated]: "
 
-/*
- * Dummy printk for disabled debugging statements to use whilst maintaining
- * gcc's format checking.
- */
-#define no_printk(fmt, ...)				\
-({							\
-	if (0)						\
-		printk(fmt, ##__VA_ARGS__);		\
-	0;						\
-})
-
 #ifdef CONFIG_EARLY_PRINTK
 extern asmlinkage __printf(1, 2)
 void early_printk(const char *fmt, ...);
diff --git a/include/linux/printk_core.h b/include/linux/printk_core.h
index a2b8727a2873..37fc0e13fdbd 100644
--- a/include/linux/printk_core.h
+++ b/include/linux/printk_core.h
@@ -6,6 +6,17 @@
 #include <linux/kern_levels.h>
 #include <linux/linkage.h>
 
+/*
+ * Dummy printk for disabled debugging statements to use whilst maintaining
+ * gcc's format checking.
+ */
+#define no_printk(fmt, ...)				\
+({							\
+	if (0)						\
+		_printk(fmt, ##__VA_ARGS__);		\
+	0;						\
+})
+
 /* Low level printk API. Use carefully! */
 
 #ifdef CONFIG_PRINTK
-- 
2.26.2