From: Jason Wessel <jason.wessel@windriver.com>
Date: Thu, 28 Jul 2011 12:42:23 -0500
Subject: kgdb/serial: Short term workaround

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.

---
 drivers/tty/serial/8250/8250.c |    3 ++-
 include/linux/kdb.h            |    2 ++
 kernel/debug/kdb/kdb_io.c      |    6 ++----
 3 files changed, 6 insertions(+), 5 deletions(-)

Index: linux-stable/drivers/tty/serial/8250/8250.c
===================================================================
--- linux-stable.orig/drivers/tty/serial/8250/8250.c
+++ linux-stable/drivers/tty/serial/8250/8250.c
@@ -38,6 +38,7 @@
 #include <linux/nmi.h>
 #include <linux/mutex.h>
 #include <linux/slab.h>
+#include <linux/kdb.h>
 #ifdef CONFIG_SPARC
 #include <linux/sunserialcore.h>
 #endif
@@ -2782,7 +2783,7 @@ serial8250_console_write(struct console 
 
 	touch_nmi_watchdog();
 
-	if (port->sysrq || oops_in_progress)
+	if (port->sysrq || oops_in_progress || in_kdb_printk())
 		locked = spin_trylock_irqsave(&port->lock, flags);
 	else
 		spin_lock_irqsave(&port->lock, flags);
Index: linux-stable/include/linux/kdb.h
===================================================================
--- linux-stable.orig/include/linux/kdb.h
+++ linux-stable/include/linux/kdb.h
@@ -148,12 +148,14 @@ extern int kdb_register(char *, kdb_func
 extern int kdb_register_repeat(char *, kdb_func_t, char *, char *,
 			       short, kdb_repeat_t);
 extern int kdb_unregister(char *);
+#define in_kdb_printk() (kdb_trap_printk)
 #else /* ! CONFIG_KGDB_KDB */
 #define kdb_printf(...)
 #define kdb_init(x)
 #define kdb_register(...)
 #define kdb_register_repeat(...)
 #define kdb_uregister(x)
+#define in_kdb_printk() (0)
 #endif	/* CONFIG_KGDB_KDB */
 enum {
 	KDB_NOT_INITIALIZED,
Index: linux-stable/kernel/debug/kdb/kdb_io.c
===================================================================
--- linux-stable.orig/kernel/debug/kdb/kdb_io.c
+++ linux-stable/kernel/debug/kdb/kdb_io.c
@@ -554,7 +554,6 @@ int vkdb_printf(const char *fmt, va_list
 	int linecount;
 	int colcount;
 	int logging, saved_loglevel = 0;
-	int saved_trap_printk;
 	int got_printf_lock = 0;
 	int retlen = 0;
 	int fnd, len;
@@ -565,8 +564,6 @@ int vkdb_printf(const char *fmt, va_list
 	unsigned long uninitialized_var(flags);
 
 	preempt_disable();
-	saved_trap_printk = kdb_trap_printk;
-	kdb_trap_printk = 0;
 
 	/* Serialize kdb_printf if multiple cpus try to write at once.
 	 * But if any cpu goes recursive in kdb, just print the output,
@@ -833,7 +830,6 @@ kdb_print_out:
 	} else {
 		__release(kdb_printf_lock);
 	}
-	kdb_trap_printk = saved_trap_printk;
 	preempt_enable();
 	return retlen;
 }
@@ -843,9 +839,11 @@ int kdb_printf(const char *fmt, ...)
 	va_list ap;
 	int r;
 
+	kdb_trap_printk++;
 	va_start(ap, fmt);
 	r = vkdb_printf(fmt, ap);
 	va_end(ap);
+	kdb_trap_printk--;
 
 	return r;
 }