[PATCH 09/10] watchdog/hardlockup: Move SMP barriers from common code to buddy code

[Douglas Anderson]

It's been suggested that since the SMP barriers are only potentially
useful for the buddy hardlockup detector, not the perf hardlockup
detector, that the barriers belong in the buddy code. Let's move them
and add clearer comments about why they're needed.

Suggested-by: Petr Mladek <pmladek at suse.com>
Signed-off-by: Douglas Anderson <dianders at chromium.org>
---

 kernel/watchdog.c       |  6 ------
 kernel/watchdog_buddy.c | 21 +++++++++++++++++++++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 6cc46b8e3d07..a351ab0c35eb 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -109,9 +109,6 @@ EXPORT_SYMBOL(arch_touch_nmi_watchdog);
 void watchdog_hardlockup_touch_cpu(unsigned int cpu)
 {
 	per_cpu(watchdog_hardlockup_touched, cpu) = true;
-
-	/* Match with smp_rmb() in watchdog_hardlockup_check() */
-	smp_wmb();
 }
 
 static bool is_hardlockup(unsigned int cpu)
@@ -141,9 +138,6 @@ static void watchdog_hardlockup_kick(void)
 
 void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 {
-	/* Match with smp_wmb() in watchdog_hardlockup_touch_cpu() */
-	smp_rmb();
-
 	if (per_cpu(watchdog_hardlockup_touched, cpu)) {
 		per_cpu(watchdog_hardlockup_touched, cpu) = false;
 		return;
diff --git a/kernel/watchdog_buddy.c b/kernel/watchdog_buddy.c
index 2ef88722c5e7..34dbfe091f4b 100644
--- a/kernel/watchdog_buddy.c
+++ b/kernel/watchdog_buddy.c
@@ -51,6 +51,13 @@ void watchdog_hardlockup_enable(unsigned int cpu)
 	if (next_cpu < nr_cpu_ids)
 		watchdog_hardlockup_touch_cpu(next_cpu);
 
+	/*
+	 * Makes sure that watchdog is touched on this CPU before
+	 * other CPUs could see it in watchdog_cpus. The counter
+	 * part is in watchdog_buddy_check_hardlockup().
+	 */
+	smp_wmb();
+
 	cpumask_set_cpu(cpu, &watchdog_cpus);
 }
 
@@ -68,6 +75,13 @@ void watchdog_hardlockup_disable(unsigned int cpu)
 	if (next_cpu < nr_cpu_ids)
 		watchdog_hardlockup_touch_cpu(next_cpu);
 
+	/*
+	 * Makes sure that watchdog is touched on the next CPU before
+	 * this CPU disappear in watchdog_cpus. The counter part is in
+	 * watchdog_buddy_check_hardlockup().
+	 */
+	smp_wmb();
+
 	cpumask_clear_cpu(cpu, &watchdog_cpus);
 }
 
@@ -88,5 +102,12 @@ void watchdog_buddy_check_hardlockup(int hrtimer_interrupts)
 	if (next_cpu >= nr_cpu_ids)
 		return;
 
+	/*
+	 * Make sure that the watchdog was touched on next CPU when
+	 * watchdog_next_cpu() returned another one because of
+	 * a change in watchdog_hardlockup_enable()/disable().
+	 */
+	smp_rmb();
+
 	watchdog_hardlockup_check(next_cpu, NULL);
 }
-- 
2.41.0.rc0.172.g3f132b7071-goog