Commit 9f113bf7 authored Apr 29, 2021 by Petr Mladek Committed by Linus Torvalds Apr 30, 2021

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@suse.com: 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@suse.com


Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 1bc503cb

kernel/watchdog.c

+6 −11

Original line number	Diff line number	Diff line
		@@ -410,11 +410,12 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
		if (kvm_check_and_clear_guest_paused())
		return HRTIMER_RESTART;

		if (softlockup_all_cpu_backtrace) {
		/* Prevent multiple soft-lockup reports if one cpu is already
		* engaged in dumping cpu back traces
		/*
		* Prevent multiple soft-lockup reports if one cpu is already
		* engaged in dumping all cpu back traces.
		*/
		if (test_and_set_bit(0, &soft_lockup_nmi_warn))
		if (softlockup_all_cpu_backtrace) {
		if (test_and_set_bit_lock(0, &soft_lockup_nmi_warn))
		return HRTIMER_RESTART;
		}

		@@ -432,14 +433,8 @@ 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();

		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);