From: Jason Wessel <jason.wessel@windriver.com>
Date: Thu, 28 Jul 2011 12:42:23 -0500
Subject: kgdb/serial: Short term workaround
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 3b763a9c24f3d7ef26a20038032f0c185e9f2a4b
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

On 07/27/2011 04:37 PM, Thomas Gleixner wrote:
>  - KGDB (not yet disabled) is reportedly unusable on -rt right now due
>    to missing hacks in the console locking which I dropped on purpose.
>

To work around this in the short term you can use this patch, in
addition to the clocksource watchdog patch that Thomas brewed up.

Comments are welcome of course.  Ultimately the right solution is to
change separation between the console and the HW to have a polled mode
+ work queue so as not to introduce any kind of latency.

Thanks,
Jason.

Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 drivers/tty/serial/8250/8250_port.c |    3 +++
 include/linux/kdb.h                 |    2 ++
 kernel/debug/kdb/kdb_io.c           |    2 ++
 3 files changed, 7 insertions(+)

--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -35,6 +35,7 @@
 #include <linux/nmi.h>
 #include <linux/mutex.h>
 #include <linux/slab.h>
+#include <linux/kdb.h>
 #include <linux/uaccess.h>
 #include <linux/pm_runtime.h>
 #include <linux/timer.h>
@@ -3181,6 +3182,8 @@ void serial8250_console_write(struct uar
 
 	if (port->sysrq || oops_in_progress)
 		locked = 0;
+	else if (in_kdb_printk())
+		locked = spin_trylock_irqsave(&port->lock, flags);
 	else
 		spin_lock_irqsave(&port->lock, flags);
 
--- a/include/linux/kdb.h
+++ b/include/linux/kdb.h
@@ -167,6 +167,7 @@ extern __printf(2, 0) int vkdb_printf(en
 extern __printf(1, 2) int kdb_printf(const char *, ...);
 typedef __printf(1, 2) int (*kdb_printf_t)(const char *, ...);
 
+#define in_kdb_printk()	(kdb_trap_printk)
 extern void kdb_init(int level);
 
 /* Access to kdb specific polling devices */
@@ -201,6 +202,7 @@ extern int kdb_register_flags(char *, kd
 extern int kdb_unregister(char *);
 #else /* ! CONFIG_KGDB_KDB */
 static inline __printf(1, 2) int kdb_printf(const char *fmt, ...) { return 0; }
+#define in_kdb_printk() (0)
 static inline void kdb_init(int level) {}
 static inline int kdb_register(char *cmd, kdb_func_t func, char *usage,
 			       char *help, short minlen) { return 0; }
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -854,9 +854,11 @@ int kdb_printf(const char *fmt, ...)
 	va_list ap;
 	int r;
 
+	kdb_trap_printk++;
 	va_start(ap, fmt);
 	r = vkdb_printf(KDB_MSGSRC_INTERNAL, fmt, ap);
 	va_end(ap);
+	kdb_trap_printk--;
 
 	return r;
 }