[RFC][PATCH RT 5/6] rt,ntp: Move call to schedule_delayed_work() to helper thread

From: Steven Rostedt
Date: Wed Jun 26 2013 - 15:33:06 EST


The ntp code for notify_cmos_timer() is called from a hard interrupt
context. schedule_delayed_work() under PREEMPT_RT_FULL calls spinlocks
that have been converted to mutexes, thus calling schedule_delayed_work()
from interrupt is not safe.

Add a helper thread that does the call to schedule_delayed_work and wake
up that thread instead of calling schedule_delayed_work() directly.
This is only for CONFIG_PREEMPT_RT_FULL, otherwise the code still calls
schedule_delayed_work() directly in irq context.

Note: There's a few places in the kernel that do this. Perhaps the RT
code should have a dedicated thread that does the checks. Just register
a notifier on boot up for your check and wake up the thread when
needed. This will be a todo.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

Index: linux-rt.git/kernel/time/ntp.c
===================================================================
--- linux-rt.git.orig/kernel/time/ntp.c
+++ linux-rt.git/kernel/time/ntp.c
@@ -10,6 +10,7 @@
#include <linux/workqueue.h>
#include <linux/hrtimer.h>
#include <linux/jiffies.h>
+#include <linux/kthread.h>
#include <linux/math64.h>
#include <linux/timex.h>
#include <linux/time.h>
@@ -516,10 +517,49 @@ static void sync_cmos_clock(struct work_
schedule_delayed_work(&sync_cmos_work, timespec_to_jiffies(&next));
}

+#ifdef CONFIG_PREEMPT_RT_FULL
+/*
+ * RT can not call schedule_delayed_work from real interrupt context.
+ * Need to make a thread to do the real work.
+ */
+static struct task_struct *cmos_delay_thread;
+static bool do_cmos_delay;
+
+static int run_cmos_delay(void *ignore)
+{
+ while (!kthread_should_stop()) {
+ set_current_state(TASK_INTERRUPTIBLE);
+ if (do_cmos_delay) {
+ do_cmos_delay = false;
+ schedule_delayed_work(&sync_cmos_work, 0);
+ }
+ schedule();
+ }
+ __set_current_state(TASK_RUNNING);
+ return 0;
+}
+
+static void notify_cmos_timer(void)
+{
+ do_cmos_delay = true;
+ /* Make visible before waking up process */
+ smp_wmb();
+ wake_up_process(cmos_delay_thread);
+}
+
+static __init int create_cmos_delay_thread(void)
+{
+ cmos_delay_thread = kthread_run(run_cmos_delay, NULL, "kcmosdelayd");
+ BUG_ON(!cmos_delay_thread);
+ return 0;
+}
+early_initcall(create_cmos_delay_thread);
+#else
static void notify_cmos_timer(void)
{
schedule_delayed_work(&sync_cmos_work, 0);
}
+#endif /* CONFIG_PREEMPT_RT_FULL */

#else
static inline void notify_cmos_timer(void) { }

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