[PATCH] printk: suppress rcu stall warnings caused by slow console devices

[Wander Lairson Costa <wander@redhat.com>]

If we have a reasonable large dataset to flush in the printk ring
buffer in the presence of a slow console device (like a serial port
with a low baud rate configured), the RCU stall detector may report
warnings.

This patch suppresses RCU stall warnings while flushing the ring buffer
to the console.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
---
 kernel/printk/printk.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 142a58d124d9..cb995f8d4f8a 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2523,6 +2523,20 @@ static inline int can_use_console(void)
 	return cpu_online(raw_smp_processor_id()) || have_callable_console();
 }
 
+extern int rcu_cpu_stall_suppress;
+
+static void rcu_console_stall_suppress(void)
+{
+	if (!rcu_cpu_stall_suppress)
+		rcu_cpu_stall_suppress = 4;
+}
+
+static void rcu_console_stall_unsuppress(void)
+{
+	if (rcu_cpu_stall_suppress == 4)
+		rcu_cpu_stall_suppress = 0;
+}
+
 /**
  * console_unlock - unlock the console system
  *
@@ -2568,6 +2582,9 @@ void console_unlock(void)
 	 * and cleared after the "again" goto label.
 	 */
 	do_cond_resched = console_may_schedule;
+
+	rcu_console_stall_suppress();
+
 again:
 	console_may_schedule = 0;
 
@@ -2579,6 +2596,7 @@ void console_unlock(void)
 	if (!can_use_console()) {
 		console_locked = 0;
 		up_console_sem();
+		rcu_console_stall_unsuppress();
 		return;
 	}
 
@@ -2645,6 +2663,7 @@ void console_unlock(void)
 
 		if (console_lock_spinning_disable_and_check()) {
 			printk_safe_exit_irqrestore(flags);
+			rcu_console_stall_unsuppress();
 			return;
 		}
 
@@ -2669,6 +2688,8 @@ void console_unlock(void)
 
 	if (retry && console_trylock())
 		goto again;
+
+	rcu_console_stall_unsuppress();
 }
 EXPORT_SYMBOL(console_unlock);
 
-- 
2.27.0