rcu: Provide API to suppress stall warnings while sysrc runs

Some sysrq handlers can run for a long time, because they dump a lot
of data onto a serial console. Having RCU stall warnings pop up in
the middle of them only makes the problem worse.

This commit provides rcu_sysrq_start() and rcu_sysrq_end() APIs to
temporarily suppress RCU CPU stall warnings while a sysrq request is
handled.

Signed-off-by: Rik van Riel <riel@redhat.com>
[ paulmck: Fix TINY_RCU build error. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcupdate.h

@@ -248,6 +248,18 @@ void rcu_idle_exit(void);
 void rcu_irq_enter(void);
 void rcu_irq_exit(void);
 
+#ifdef CONFIG_RCU_STALL_COMMON
+void rcu_sysrq_start(void);
+void rcu_sysrq_end(void);
+#else /* #ifdef CONFIG_RCU_STALL_COMMON */
+static inline void rcu_sysrq_start(void)
+{
+}
+static inline void rcu_sysrq_end(void)
+{
+}
+#endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
+
 #ifdef CONFIG_RCU_USER_QS
 void rcu_user_enter(void);
 void rcu_user_exit(void);

kernel/rcu/update.c

@@ -320,6 +320,18 @@ int rcu_jiffies_till_stall_check(void)
 	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
 }
 
+void rcu_sysrq_start(void)
+{
+	if (!rcu_cpu_stall_suppress)
+		rcu_cpu_stall_suppress = 2;
+}
+
+void rcu_sysrq_end(void)
+{
+	if (rcu_cpu_stall_suppress == 2)
+		rcu_cpu_stall_suppress = 0;
+}
+
 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
 {
 	rcu_cpu_stall_suppress = 1;