+ watchdog-fix-barriers-when-printing-backtraces-from-all-cpus.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: watchdog: fix barriers when printing backtraces from all CPUs
has been added to the -mm tree.  Its filename is
     watchdog-fix-barriers-when-printing-backtraces-from-all-cpus.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/watchdog-fix-barriers-when-printing-backtraces-from-all-cpus.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/watchdog-fix-barriers-when-printing-backtraces-from-all-cpus.patch

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------------------------------------------------------
From: Petr Mladek <pmladek@suse.com>
Subject: 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.

Link: https://lkml.kernel.org/r/20210311122130.6788-6-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/watchdog.c |   24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

--- a/kernel/watchdog.c~watchdog-fix-barriers-when-printing-backtraces-from-all-cpus
+++ a/kernel/watchdog.c
@@ -409,12 +409,18 @@ static enum hrtimer_restart watchdog_tim
 		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_tim
 			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);
_

Patches currently in -mm which might be from pmladek@suse.com are

watchdog-rename-__touch_watchdog-to-a-better-descriptive-name.patch
watchdog-explicitly-update-timestamp-when-reporting-softlockup.patch
watchdog-softlockup-report-the-overall-time-of-softlockups.patch
watchdog-softlockup-remove-logic-that-tried-to-prevent-repeated-reports.patch
watchdog-fix-barriers-when-printing-backtraces-from-all-cpus.patch
watchdog-cleanup-handling-of-false-positives.patch