4.18-stable review patch. If anyone has any objections, please let me know.
------------------
From: Peter Zijlstra <pet...@infradead.org>
commit e2c631ba75a7e727e8db0a9d30a06bfd434adb3a upstream.
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: 7197e77abcb6 ("clocksource: Remove kthread")
Reported-by: Siegfried Metz <fr...@mailbox.org>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Tested-by: Niklas Cassel <niklas.cas...@linaro.org>
Tested-by: Kevin Shanahan <ke...@shanahan.id.au>
Tested-by: viktor_jaegerskuep...@freenet.de
Tested-by: Siegfried Metz <fr...@mailbox.org>
Cc: rafael.j.wyso...@intel.com
Cc: len.br...@intel.com
Cc: diego.vi...@gmail.com
Cc: rui.zh...@intel.com
Cc: bjorn.anders...@linaro.org
Link:
https://lkml.kernel.org/r/20180905084158.gr24...@hirez.programming.kicks-ass.net
Cc: Siegfried Metz <fr...@mailbox.org>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
kernel/time/clocksource.c | 40 ++++++++++++++++++++++++++++++----------
1 file changed, 30 insertions(+), 10 deletions(-)
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -129,19 +129,40 @@ static void inline clocksource_watchdog_
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)) {
@@ -152,7 +173,7 @@ static void __clocksource_unstable(struc
if (cs->mark_unstable)
cs->mark_unstable(cs);
- /* kick clocksource_watchdog_work() */
+ /* kick clocksource_watchdog_kthread() */
if (finished_booting)
schedule_work(&watchdog_work);
}
@@ -162,7 +183,7 @@ static void __clocksource_unstable(struc
* @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)
{
@@ -387,9 +408,7 @@ static void clocksource_dequeue_watchdog
}
}
-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;
@@ -414,12 +433,13 @@ static int __clocksource_watchdog_work(v
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)
@@ -438,7 +458,7 @@ static void clocksource_enqueue_watchdog
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) { }
@@ -672,7 +692,7 @@ static int __init clocksource_done_booti
/*
* Run the watchdog first to eliminate unstable clock sources
*/
- __clocksource_watchdog_work();
+ __clocksource_watchdog_kthread();
clocksource_select();
mutex_unlock(&clocksource_mutex);
return 0;
