clocksource: Revert "Remove kthread"

I turns out that the silly spawn kthread from worker was actually needed.

clocksource_watchdog_kthread() cannot be called directly from
clocksource_watchdog_work(), because clocksource_select() calls
timekeeping_notify() which uses stop_machine(). One cannot use
stop_machine() from a workqueue() due lock inversions wrt CPU hotplug.

Revert the patch but add a comment that explain why we jump through such
apparently silly hoops.

Fixes: 99d7f19ce658 ("clocksource: Remove kthread")
Reported-by: Siegfried Metz <frame@mailbox.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Niklas Cassel <niklas.cassel@linaro.org>
Tested-by: Kevin Shanahan <kevin@shanahan.id.au>
Tested-by: viktor_jaegerskuepper@freenet.de
Tested-by: Siegfried Metz <frame@mailbox.org>
Cc: rafael.j.wysocki@intel.com
Cc: len.brown@intel.com
Cc: diego.viola@gmail.com
Cc: rui.zhang@intel.com
Cc: bjorn.andersson@linaro.org
Link: https://lkml.kernel.org/r/20180905084158.GR24124@hirez.programming.kicks-ass.net

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index f74fb00d806444739f9d8ee1611a98c694325f95..0e6e97a01942d956e6135025003371537ca28ab0 100644 (file)
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -133,19 +133,40 @@ static void inline clocksource_watchdog_unlock(unsigned long *flags)
        spin_unlock_irqrestore(&watchdog_lock, *flags);
 }
 
+static int clocksource_watchdog_kthread(void *data);
+static void __clocksource_change_rating(struct clocksource *cs, int rating);
+
 /*
  * Interval: 0.5sec Threshold: 0.0625s
  */
 #define WATCHDOG_INTERVAL (HZ >> 1)
 #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
 
+static void clocksource_watchdog_work(struct work_struct *work)
+{
+       /*
+        * We cannot directly run clocksource_watchdog_kthread() here, because
+        * clocksource_select() calls timekeeping_notify() which uses
+        * stop_machine(). One cannot use stop_machine() from a workqueue() due
+        * lock inversions wrt CPU hotplug.
+        *
+        * Also, we only ever run this work once or twice during the lifetime
+        * of the kernel, so there is no point in creating a more permanent
+        * kthread for this.
+        *
+        * If kthread_run fails the next watchdog scan over the
+        * watchdog_list will find the unstable clock again.
+        */
+       kthread_run(clocksource_watchdog_kthread, NULL, "kwatchdog");
+}
+
 static void __clocksource_unstable(struct clocksource *cs)
 {
        cs->flags &= ~(CLOCK_SOURCE_VALID_FOR_HRES | CLOCK_SOURCE_WATCHDOG);
        cs->flags |= CLOCK_SOURCE_UNSTABLE;
 
        /*
-        * If the clocksource is registered clocksource_watchdog_work() will
+        * If the clocksource is registered clocksource_watchdog_kthread() will
         * re-rate and re-select.
         */
        if (list_empty(&cs->list)) {
@@ -156,7 +177,7 @@ static void __clocksource_unstable(struct clocksource *cs)
        if (cs->mark_unstable)
                cs->mark_unstable(cs);
 
-       /* kick clocksource_watchdog_work() */
+       /* kick clocksource_watchdog_kthread() */
        if (finished_booting)
                schedule_work(&watchdog_work);
 }
@@ -166,7 +187,7 @@ static void __clocksource_unstable(struct clocksource *cs)
  * @cs:                clocksource to be marked unstable
  *
  * This function is called by the x86 TSC code to mark clocksources as unstable;
- * it defers demotion and re-selection to a work.
+ * it defers demotion and re-selection to a kthread.
  */
 void clocksource_mark_unstable(struct clocksource *cs)
 {
@@ -391,9 +412,7 @@ static void clocksource_dequeue_watchdog(struct clocksource *cs)
        }
 }
 
-static void __clocksource_change_rating(struct clocksource *cs, int rating);
-
-static int __clocksource_watchdog_work(void)
+static int __clocksource_watchdog_kthread(void)
 {
        struct clocksource *cs, *tmp;
        unsigned long flags;
@@ -418,12 +437,13 @@ static int __clocksource_watchdog_work(void)
        return select;
 }
 
-static void clocksource_watchdog_work(struct work_struct *work)
+static int clocksource_watchdog_kthread(void *data)
 {
        mutex_lock(&clocksource_mutex);
-       if (__clocksource_watchdog_work())
+       if (__clocksource_watchdog_kthread())
                clocksource_select();
        mutex_unlock(&clocksource_mutex);
+       return 0;
 }
 
 static bool clocksource_is_watchdog(struct clocksource *cs)
@@ -442,7 +462,7 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
 static void clocksource_select_watchdog(bool fallback) { }
 static inline void clocksource_dequeue_watchdog(struct clocksource *cs) { }
 static inline void clocksource_resume_watchdog(void) { }
-static inline int __clocksource_watchdog_work(void) { return 0; }
+static inline int __clocksource_watchdog_kthread(void) { return 0; }
 static bool clocksource_is_watchdog(struct clocksource *cs) { return false; }
 void clocksource_mark_unstable(struct clocksource *cs) { }
 
@@ -810,7 +830,7 @@ static int __init clocksource_done_booting(void)
        /*
         * Run the watchdog first to eliminate unstable clock sources
         */
-       __clocksource_watchdog_work();
+       __clocksource_watchdog_kthread();
        clocksource_select();
        mutex_unlock(&clocksource_mutex);
        return 0;