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

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Message-Id: <20201210160038.31441-6-pmladek@suse.com>
Date:   Thu, 10 Dec 2020 17:00:36 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Laurence Oberman <loberman@...hat.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        Petr Mladek <pmladek@...e.com>
Subject: [PATCH v2 5/7] watchdog: Fix barriers when printing backtraces from all CPUs

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@...e.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