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 <[email protected]> --- 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
