printk: Remove superfluous memory barriers from printk_safe

The variable printk_safe_irq_ready is set and never cleared at system
boot up, when there's only one CPU active. It is set before other
CPUs come on line. Also, it is extremely unlikely that an NMI would
trigger this early in boot up (which I wonder why we even have this
variable at all).

Also mark the printk_safe_irq_ready as read mostly, as it is set at
system boot up, and never touched again.

Link: http://lkml.kernel.org/r/20171011124647.7781f98f@gandalf.local.home

Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/printk/printk_safe.c

@@ -39,7 +39,7 @@
  * There are situations when we want to make sure that all buffers
  * were handled or when IRQs are blocked.
  */
-static int printk_safe_irq_ready;
+static int printk_safe_irq_ready __read_mostly;
 
 #define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
 				sizeof(atomic_t) -			\
@@ -63,11 +63,8 @@ static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
 /* Get flushed in a more safe context. */
 static void queue_flush_work(struct printk_safe_seq_buf *s)
 {
-	if (printk_safe_irq_ready) {
-		/* Make sure that IRQ work is really initialized. */
-		smp_rmb();
+	if (printk_safe_irq_ready)
 		irq_work_queue(&s->work);
-	}
 }
 
 /*
@@ -398,8 +395,12 @@ void __init printk_safe_init(void)
 #endif
 	}
 
-	/* Make sure that IRQ works are initialized before enabling. */
-	smp_wmb();
+	/*
+	 * In the highly unlikely event that a NMI were to trigger at
+	 * this moment. Make sure IRQ work is set up before this
+	 * variable is set.
+	 */
+	barrier();
 	printk_safe_irq_ready = 1;
 
 	/* Flush pending messages that did not have scheduled IRQ works. */