In order to avoid multiple CPUs banging on the serial port at the same
time, add simple serialization. This explicitly deals with nested
contexts (like IRQs etc.).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/printk/printk.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -356,14 +356,28 @@ static int __init force_early_printk_set
 }
 early_param("force_early_printk", force_early_printk_setup);
 
+static int early_printk_cpu = -1;
+
 static int early_vprintk(const char *fmt, va_list args)
 {
+	int n, cpu, old;
 	char buf[512];
-	int n;
+
+	cpu = get_cpu();
+	for (;;) {
+		old = cmpxchg(&early_printk_cpu, -1, cpu);
+		if (old == -1 || old == cpu)
+			break;
+
+		cpu_relax();
+	}
 
 	n = vscnprintf(buf, sizeof(buf), fmt, args);
 	early_console->write(early_console, buf, n);
 
+	smp_store_release(&early_printk_cpu, old);
+	put_cpu();
+
 	return n;
 }