[PATCH v4 2/5] printk/nmi: use IRQ work only when ready

[Petr Mladek]

NMIs could happen at any time.  This patch makes sure that the safe
printk() in NMI will schedule IRQ work only when the related structs are
initialized.

All pending messages are flushed when the IRQ work is being initialized.

Signed-off-by: Petr Mladek <pmladek at suse.com>
Cc: Jan Kara <jack at suse.cz>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Steven Rostedt <rostedt at goodmis.org>
Cc: Russell King <rmk+kernel at arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson at linaro.org>
Cc: Jiri Kosina <jkosina at suse.com>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Ralf Baechle <ralf at linux-mips.org>
Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky at de.ibm.com>
Cc: David Miller <davem at davemloft.net>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
---
 kernel/printk/nmi.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 479e0764203c..303cf0d15e57 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -38,6 +38,7 @@
  * were handled or when IRQs are blocked.
  */
 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
+static int printk_nmi_irq_ready;
 
 #define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
 
@@ -84,8 +85,11 @@ again:
 		goto again;
 
 	/* Get flushed in a more safe context. */
-	if (add)
+	if (add && printk_nmi_irq_ready) {
+		/* Make sure that IRQ work is really initialized. */
+		smp_rmb();
 		irq_work_queue(&s->work);
+	}
 
 	return add;
 }
@@ -195,6 +199,13 @@ void __init printk_nmi_init(void)
 
 		init_irq_work(&s->work, __printk_nmi_flush);
 	}
+
+	/* Make sure that IRQ works are initialized before enabling. */
+	smp_wmb();
+	printk_nmi_irq_ready = 1;
+
+	/* Flush pending messages that did not have scheduled IRQ works. */
+	printk_nmi_flush();
 }
 
 void printk_nmi_enter(void)
-- 
1.8.5.6