[PATCH] clocksource: Allow unregistering the watchdog

[PATCH] clocksource: Allow unregistering the watchdog

[Vitaly Kuznetsov]

Hyper-V vmbus module register TSC page clocksource when loaded. This is the
clocksource with the highest rating and thus it becomes the watchdog making
unloading of the vmbus module impossible.
Separate clocksource_select_watchdog() from clocksource_enqueue_watchdog()
and use it on clocksource rating change/unregister.

After all, lobotomized monkeys may need some love too.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 kernel/time/clocksource.c | 52 ++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 42 insertions(+), 10 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 664de53..51e71e4 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -323,13 +323,43 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
 		/* cs is a watchdog. */
 		if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS)
 			cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
+	}
+	spin_unlock_irqrestore(&watchdog_lock, flags);
+}
+
+static void clocksource_select_watchdog(bool fallback)
+{
+	unsigned long flags;
+	struct clocksource *cs;
+	struct clocksource *old_wd;
+
+	spin_lock_irqsave(&watchdog_lock, flags);
+	/* save current watchdog */
+	old_wd = watchdog;
+	if (fallback)
+		watchdog = NULL;
+
+	list_for_each_entry(cs, &clocksource_list, list) {
+		/* cs is a clocksource to be watched. */
+		if (cs->flags & CLOCK_SOURCE_MUST_VERIFY)
+			continue;
+
+		/* Skip current if we were requested for a fallback. */
+		if (fallback && cs == old_wd)
+			continue;
+
 		/* Pick the best watchdog. */
-		if (!watchdog || cs->rating > watchdog->rating) {
+		if (!watchdog || cs->rating > watchdog->rating)
 			watchdog = cs;
-			/* Reset watchdog cycles */
-			clocksource_reset_watchdog();
-		}
 	}
+	/* If we failed to find a fallback restore the old one. */
+	if (!watchdog)
+		watchdog = old_wd;
+
+	/* If we changed the watchdog we need to reset cycles. */
+	if (watchdog != old_wd)
+		clocksource_reset_watchdog();
+
 	/* Check if the watchdog timer needs to be started. */
 	clocksource_start_watchdog();
 	spin_unlock_irqrestore(&watchdog_lock, flags);
@@ -736,6 +766,7 @@ int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
 	clocksource_enqueue(cs);
 	clocksource_enqueue_watchdog(cs);
 	clocksource_select();
+	clocksource_select_watchdog(false);
 	mutex_unlock(&clocksource_mutex);
 	return 0;
 }
@@ -758,6 +789,7 @@ void clocksource_change_rating(struct clocksource *cs, int rating)
 	mutex_lock(&clocksource_mutex);
 	__clocksource_change_rating(cs, rating);
 	clocksource_select();
+	clocksource_select_watchdog(false);
 	mutex_unlock(&clocksource_mutex);
 }
 EXPORT_SYMBOL(clocksource_change_rating);
@@ -767,12 +799,12 @@ EXPORT_SYMBOL(clocksource_change_rating);
  */
 static int clocksource_unbind(struct clocksource *cs)
 {
-	/*
-	 * I really can't convince myself to support this on hardware
-	 * designed by lobotomized monkeys.
-	 */
-	if (clocksource_is_watchdog(cs))
-		return -EBUSY;
+	if (clocksource_is_watchdog(cs)) {
+		/* Select and try to install a replacement watchdog. */
+		clocksource_select_watchdog(true);
+		if (clocksource_is_watchdog(cs))
+			return -EBUSY;
+	}
 
 	if (cs == curr_clocksource) {
 		/* Select and try to install a replacement clock source */
-- 
2.5.0

Re: [PATCH] clocksource: Allow unregistering the watchdog

[kbuild test robot]

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Hi Vitaly,

[auto build test ERROR on tip/timers/core]
[also build test ERROR on v4.4 next-20160122]
[if your patch is applied to the wrong git tree, please drop us a note to help improving the system]

url:    https://github.com/0day-ci/linux/commits/Vitaly-Kuznetsov/clocksource-Allow-unregistering-the-watchdog/20160123-010529
config: xtensa-allyesconfig (attached as .config)
reproduce:
        wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=xtensa 

All errors (new ones prefixed by >>):

   kernel/time/clocksource.c: In function '__clocksource_register_scale':
>> kernel/time/clocksource.c:769:2: error: implicit declaration of function 'clocksource_select_watchdog' [-Werror=implicit-function-declaration]
     clocksource_select_watchdog(false);
     ^
   cc1: some warnings being treated as errors

vim +/clocksource_select_watchdog +769 kernel/time/clocksource.c

   763	
   764		/* Add clocksource to the clocksource list */
   765		mutex_lock(&clocksource_mutex);
   766		clocksource_enqueue(cs);
   767		clocksource_enqueue_watchdog(cs);
   768		clocksource_select();
 > 769		clocksource_select_watchdog(false);
   770		mutex_unlock(&clocksource_mutex);
   771		return 0;
   772	}

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

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