From: Petr Mladek <pmladek@xxxxxxxx> 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. [pmladek@xxxxxxxx: use bit lock operations to prevent multiple soft-lockup reports] Link: https://lkml.kernel.org/r/YFSVsLGVWMXTvlbk@alley Link: https://lkml.kernel.org/r/20210311122130.6788-6-pmladek@xxxxxxxx Signed-off-by: Petr Mladek <pmladek@xxxxxxxx> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Laurence Oberman <loberman@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- kernel/watchdog.c | 17 ++++++----------- 1 file changed, 6 insertions(+), 11 deletions(-) --- a/kernel/watchdog.c~watchdog-fix-barriers-when-printing-backtraces-from-all-cpus +++ a/kernel/watchdog.c @@ -410,11 +410,12 @@ 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)) + if (test_and_set_bit_lock(0, &soft_lockup_nmi_warn)) return HRTIMER_RESTART; } @@ -432,14 +433,8 @@ 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(); - - clear_bit(0, &soft_lockup_nmi_warn); - /* Barrier to sync with other cpus */ - smp_mb__after_atomic(); + clear_bit_unlock(0, &soft_lockup_nmi_warn); } add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK); _
