[PATCH v2] Add hard/soft lockup debugger entry points

[Jeff Merkey <linux.mdb@gmail.com>]

This patch adds an export which can be set by system debuggers to direct
the hard lockup and soft lockup detector to trigger a breakpoint exception
and enter a debugger if one is active.  It is assumed that if someone
sets this variable, then an breakpoint handler of some sort will be actively
loaded or registered via the notify die handler chain.

This addition is extremely useful for debugging hard and soft lockups
real time and quickly from a console debugger.

Signed-off-by: Jeff Merkey <linux.mdb@gmail.com>
---
 kernel/watchdog.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index b3ace6e..c28e58c 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -23,6 +23,7 @@
 #include <linux/workqueue.h>
 
 #include <asm/irq_regs.h>
+#include <linux/kgdb.h>
 #include <linux/kvm_para.h>
 #include <linux/perf_event.h>
 #include <linux/kthread.h>
@@ -108,6 +109,9 @@ static DEFINE_PER_CPU(struct perf_event *, watchdog_ev);
 #endif
 static unsigned long soft_lockup_nmi_warn;
 
+int debug_watchdog_lockups;
+EXPORT_SYMBOL_GPL(debug_watchdog_lockups);
+
 /* boot commands */
 /*
  * Should we panic when a soft-lockup or hard-lockup occurs:
@@ -358,6 +362,9 @@ static void watchdog_overflow_callback(struct perf_event *event,
 		else
 			dump_stack();
 
+		if (debug_watchdog_lockups)
+			arch_kgdb_breakpoint();
+
 		/*
 		 * Perform all-CPU dump only once to avoid multiple hardlockups
 		 * generating interleaving traces
@@ -478,6 +485,9 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 		else
 			dump_stack();
 
+		if (debug_watchdog_lockups)
+			arch_kgdb_breakpoint();
+
 		if (softlockup_all_cpu_backtrace) {
 			/* Avoid generating two back traces for current
 			 * given that one is already made above
-- 
1.8.3.1