printk: Don't take console semaphore in atomic context

The CPU HOTPLUG take_cpu_down path is invokved with preemption
disabled via stop_machine. This causes a "Scheduling while
atomic" BUG when there is contention for the console semaphore.
The solution is to defer the console flush until it's not in
scheduling violation.

Change-Id: I2d0d58576a4db308ee40850a18a6bb9784ca4e4b
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
(cherry picked from commit f6d11b2eb9c110d0801aa40b1bfdb8194a5e3e3a)

kernel/printk.c

@@ -1144,6 +1144,14 @@ void resume_console(void)
 	console_unlock();
 }
 
+static void __cpuinit console_flush(struct work_struct *work)
+{
+	console_lock();
+	console_unlock();
+}
+
+static __cpuinitdata DECLARE_WORK(console_cpu_notify_work, console_flush);
+
 /**
  * console_cpu_notify - print deferred console messages after CPU hotplug
  * @self: notifier struct
@@ -1154,6 +1162,9 @@ void resume_console(void)
  * will be spooled but will not show up on the console.  This function is
  * called when a new CPU comes online (or fails to come up), and ensures
  * that any such output gets printed.
+ *
+ * Special handling must be done for cases invoked from an atomic context,
+ * as we can't be taking the console semaphore here.
  */
 static int __cpuinit console_cpu_notify(struct notifier_block *self,
 	unsigned long action, void *hcpu)
@@ -1161,11 +1172,16 @@ static int __cpuinit console_cpu_notify(struct notifier_block *self,
 	switch (action) {
 	case CPU_ONLINE:
 	case CPU_DEAD:
-	case CPU_DYING:
 	case CPU_DOWN_FAILED:
 	case CPU_UP_CANCELED:
 		console_lock();
 		console_unlock();
+	/* invoked with preemption disabled, so defer */
+	case CPU_DYING:
+		if (!console_trylock())
+			schedule_work(&console_cpu_notify_work);
+		else
+			console_unlock();
 	}
 	return NOTIFY_OK;
 }