[RFC PATCH 2/2] rcu,debug_core: allow the kernel debugger to reset the rcu stall timer

[Jason Wessel <jason.wessel@windriver.com>]

When returning from the kernel debugger allow a reset of the rcu
jiffies_stall value to prevent the rcu stall detector from sending NMI
events which stack dumps on all the cpus in the system.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
CC: Dipankar Sarma <dipankar@in.ibm.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Ingo Molnar <mingo@elte.hu>
---
 include/linux/rcupdate.h  |    8 ++++++++
 kernel/debug/debug_core.c |    2 ++
 kernel/rcutree.c          |    9 +++++++++
 3 files changed, 19 insertions(+), 0 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9fbc54a..abd3ab6 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -599,4 +599,12 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 #define rcu_dereference_index_check(p, c) \
 	__rcu_dereference_index_check((p), (c))
 
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+extern void rcu_cpu_stall_reset(void);
+#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+static inline void rcu_cpu_stall_reset(void)
+{
+}
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
 #endif /* __LINUX_RCUPDATE_H */
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index e4d6819..1600e90 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -47,6 +47,7 @@
 #include <linux/pid.h>
 #include <linux/smp.h>
 #include <linux/mm.h>
+#include <linux/rcupdate.h>
 
 #include <asm/cacheflush.h>
 #include <asm/byteorder.h>
@@ -474,6 +475,7 @@ static void dbg_touch_watchdogs(void)
 {
 	touch_softlockup_watchdog_sync();
 	clocksource_touch_watchdog();
+	rcu_cpu_stall_reset();
 }
 
 static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d5bc439..209b755 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -532,6 +532,9 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
 
 	if (rcu_cpu_stall_panicking)
 		return;
+	/* Gracefully handle a watch dog reset when jiffies_stall == 0 */
+	if (!rsp->jiffies_stall)
+		return;
 	delta = jiffies - rsp->jiffies_stall;
 	rnp = rdp->mynode;
 	if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {
@@ -561,6 +564,12 @@ static void __init check_cpu_stall_init(void)
 	atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
 }
 
+void rcu_cpu_stall_reset(void)
+{
+	rcu_sched_state.jiffies_stall = 0;
+	rcu_bh_state.jiffies_stall = 0;
+}
+
 #else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
 static void record_gp_stall_check_time(struct rcu_state *rsp)
-- 
1.6.3.3