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

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Message-ID: <Yd2aQmuDqsUU99Y6@chrisdown.name>
Date:   Tue, 11 Jan 2022 14:54:58 +0000
From:   Chris Down <chris@...isdown.name>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        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,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [RFC 2/2] printk/bug: Remove cyclic dependency between bug.h and
 printk.h

Petr Mladek writes:
>`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>

Thank you! Looks good.

Acked-by: Chris Down <chris@...isdown.name>

>---
> 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
>