backtrace: replace timer with tasklet + completions

On qemu, the backtrace would show up _after_ the "end of backtrace
testing" message.

This patch changes it to use completions instead, which will guarantee
that no such race exists.

Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/backtracetest.c

@@ -10,18 +10,39 @@
  * of the License.
  */
 
+#include <linux/completion.h>
 #include <linux/delay.h>
+#include <linux/interrupt.h>
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <linux/stacktrace.h>
 
-static struct timer_list backtrace_timer;
+static void backtrace_test_normal(void)
+{
+	printk("Testing a backtrace from process context.\n");
+	printk("The following trace is a kernel self test and not a bug!\n");
+
+	dump_stack();
+}
+
+static DECLARE_COMPLETION(backtrace_work);
+
+static void backtrace_test_irq_callback(unsigned long data)
+{
+	dump_stack();
+	complete(&backtrace_work);
+}
+
+static DECLARE_TASKLET(backtrace_tasklet, &backtrace_test_irq_callback, 0);
 
-static void backtrace_test_timer(unsigned long data)
+static void backtrace_test_irq(void)
 {
 	printk("Testing a backtrace from irq context.\n");
 	printk("The following trace is a kernel self test and not a bug!\n");
-	dump_stack();
+
+	init_completion(&backtrace_work);
+	tasklet_schedule(&backtrace_tasklet);
+	wait_for_completion(&backtrace_work);
 }
 
 #ifdef CONFIG_STACKTRACE
@@ -51,17 +72,11 @@ static void backtrace_test_saved(void)
 static int backtrace_regression_test(void)
 {
 	printk("====[ backtrace testing ]===========\n");
-	printk("Testing a backtrace from process context.\n");
-	printk("The following trace is a kernel self test and not a bug!\n");
-	dump_stack();
 
+	backtrace_test_normal();
+	backtrace_test_irq();
 	backtrace_test_saved();
 
-	init_timer(&backtrace_timer);
-	backtrace_timer.function = backtrace_test_timer;
-	mod_timer(&backtrace_timer, jiffies + 10);
-
-	msleep(10);
 	printk("====[ end of backtrace testing ]====\n");
 	return 0;
 }