console: Replace #if 0 with atomic var 'ignore_console_lock_warning'

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/[email protected]
Signed-off-by: Thomas Zimmermann <[email protected]>
Acked-by: Hans de Goede <[email protected]>
Acked-by: Petr Mladek <[email protected]>
Reviewed-by: Sergey Senozhatsky <[email protected]>
Cc: Steven Rostedt (VMware) <[email protected]>
Cc: Andrew Morton <[email protected]>
[b.zolnierkie: use <linux/atomic.h>]
Signed-off-by: Bartlomiej Zolnierkiewicz <[email protected]>

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.