[PATCH v3 1/2] console: Replace #if 0 with atomic var 'ignore_console_lock_warning'

[Thomas Zimmermann]

The macro WARN_CONSOLE_UNLOCKED prints a warning when a thread enters
the console's critical section without having acquired the console
lock. The console lock can be ignored when debugging the console using
printk, but this makes WARN_CONSOLE_UNLOCKED generate unnecessary
warnings.

The variable ignore_console_lock_warning temporarily disables
WARN_CONSOLE_UNLOCKED. Developers interested in debugging the console's
critical sections should increment it before entering the CS and
decrement it after leaving the CS. Setting ignore_console_lock_warning
is only for debugging. Regular operation should not manipulate it.

Acknoledgements: This patch is based on an earlier version by Steven
Rostedt. The use of atomic increment/decrement was suggested by Petr
Mladek.

Link: http://lkml.kernel.org/r/717e6337-e7a6-7a92-1c1b-8929a25696b5@suse.de
Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 include/linux/console.h | 14 +++++++++-----
 kernel/printk/printk.c  |  3 +++
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index f59f3dbca65c..a3307bcab947 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -14,6 +14,7 @@
 #ifndef _LINUX_CONSOLE_H_
 #define _LINUX_CONSOLE_H_ 1
 
+#include <asm/atomic.h>
 #include <linux/types.h>
 
 struct vc_data;
@@ -201,11 +202,14 @@ void vcs_make_sysfs(int index);
 void vcs_remove_sysfs(int index);
 
 /* Some debug stub to catch some of the obvious races in the VT code */
-#if 1
-#define WARN_CONSOLE_UNLOCKED()	WARN_ON(!is_console_locked() && !oops_in_progress)
-#else
-#define WARN_CONSOLE_UNLOCKED()
-#endif
+#define WARN_CONSOLE_UNLOCKED()						\
+	WARN_ON(!atomic_read(&ignore_console_lock_warning) &&		\
+		!is_console_locked() && !oops_in_progress)
+/*
+ * Increment ignore_console_lock_warning if you need to quiet
+ * WARN_CONSOLE_UNLOCKED() for debugging purposes.
+ */
+extern atomic_t ignore_console_lock_warning;
 
 /* VESA Blanking Levels */
 #define VESA_NO_BLANKING        0
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3f041e7cbfc9..7d32a86758cd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -66,6 +66,9 @@ int console_printk[4] = {
 	CONSOLE_LOGLEVEL_DEFAULT,	/* default_console_loglevel */
 };
 
+atomic_t ignore_console_lock_warning __read_mostly = ATOMIC_INIT(0);
+EXPORT_SYMBOL(ignore_console_lock_warning);
+
 /*
  * Low level drivers may need that to know if they can schedule in
  * their unblank() callback or not. So let's export it.
-- 
2.18.0