watchdog/hardlockup: move SMP barriers from common code to buddy code

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.

Link: https://lkml.kernel.org/r/20230526184139.9.I5ab0a0eeb0bd52fb23f901d298c72fa5c396e22b@changeid
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Suggested-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: dianders

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;

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);
 }