[PATCH v2 5/7] watchdog: Fix barriers when printing backtraces from all CPUs

[Petr Mladek <pmladek@suse.com>]

Any parallel softlockup reports are skipped when one CPU is already
printing backtraces from all CPUs.

The exclusive rights are synchronized using one bit in
soft_lockup_nmi_warn. There is also one memory barrier that
does not make much sense.

Use two barriers on the right location to prevent mixing two
reports.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/watchdog.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index dc8a0bf943f5..6dc1f79e36aa 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -409,12 +409,18 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 		if (kvm_check_and_clear_guest_paused())
 			return HRTIMER_RESTART;
 
+		/*
+		 * Prevent multiple soft-lockup reports if one cpu is already
+		 * engaged in dumping all cpu back traces.
+		 */
 		if (softlockup_all_cpu_backtrace) {
-			/* Prevent multiple soft-lockup reports if one cpu is already
-			 * engaged in dumping cpu back traces
-			 */
 			if (test_and_set_bit(0, &soft_lockup_nmi_warn))
 				return HRTIMER_RESTART;
+			/*
+			 * Make sure that reports are serialized. Start
+			 * printing after getting the exclusive rights.
+			 */
+			smp_mb__after_atomic();
 		}
 
 		/* Start period for the next softlockup warning. */
@@ -431,14 +437,14 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 			dump_stack();
 
 		if (softlockup_all_cpu_backtrace) {
-			/* Avoid generating two back traces for current
-			 * given that one is already made above
-			 */
 			trigger_allbutself_cpu_backtrace();
-
+			/*
+			 * Make sure that everything is printed before
+			 * another CPU is allowed to report lockup again.
+			 */
+			smp_mb__before_atomic();
+			/* Allow a further report. */
 			clear_bit(0, &soft_lockup_nmi_warn);
-			/* Barrier to sync with other cpus */
-			smp_mb__after_atomic();
 		}
 
 		add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
-- 
2.26.2