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.

[[email protected]: use bit lock operations to prevent multiple soft-lockup reports]
  Link: https://lkml.kernel.org/r/YFSVsLGVWMXTvlbk@alley

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Petr Mladek <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Laurence Oberman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vincent Whitchurch <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/watchdog.c

@@ -410,11 +410,12 @@ 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))
+			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);