sysrq,rcu: suppress RCU stall warnings while sysrq 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 patch temporarily disables RCU stall warnings while a sysrq
request is handled.

Signed-off-by: Rik van Riel <[email protected]>
Suggested-by: Paul McKenney <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Joern Engel <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Madper Xie <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

drivers/tty/sysrq.c

@@ -511,6 +511,7 @@ void __handle_sysrq(int key, bool check_mask)
 	int orig_log_level;
 	int i;
 
+	rcu_sysrq_start();
 	rcu_read_lock();
 	/*
 	 * Raise the apparent loglevel to maximum so that the sysrq header
@@ -554,6 +555,7 @@ void __handle_sysrq(int key, bool check_mask)
 		console_loglevel = orig_log_level;
 	}
 	rcu_read_unlock();
+	rcu_sysrq_end();
 }
 
 void handle_sysrq(int key)

include/linux/rcupdate.h

@@ -227,6 +227,8 @@ void rcu_idle_enter(void);
 void rcu_idle_exit(void);
 void rcu_irq_enter(void);
 void rcu_irq_exit(void);
+void rcu_sysrq_start(void);
+void rcu_sysrq_end(void);
 
 #ifdef CONFIG_RCU_USER_QS
 void rcu_user_enter(void);

kernel/rcu/update.c

@@ -320,6 +320,17 @@ int rcu_jiffies_till_stall_check(void)
 	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
 }
 
+void rcu_sysrq_start(void)
+{
+	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;