[PATCH] watchdog: Use bit lock operations to prevent multiple soft-lockup reports

[Petr Mladek <pmladek@suse.com>]

Use bit_lock operation to prevent multiple soft-lockups reports when
one CPU already triggered dumping backtraces from all CPUs.

It allows to remove the explicit memory barriers and misleading
comments.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
---
Andrew,

this patch can be put on top of the patchset fixing/cleaning softlockup
watchdog code. Feel free to squash it into the other patch fixing
the barriers.

Or should I resend the entire patchset again, please?

Best Regards,
Petr

 kernel/watchdog.c | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index c050323fcd33..090b6bc4de79 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -410,13 +410,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 		 * engaged in dumping all cpu back traces.
 		 */
 		if (softlockup_all_cpu_backtrace) {
-			if (test_and_set_bit(0, &soft_lockup_nmi_warn))
+			if (test_and_set_bit_lock(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. */
@@ -434,13 +429,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 
 		if (softlockup_all_cpu_backtrace) {
 			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);
+			clear_bit_unlock(0, &soft_lockup_nmi_warn);
 		}
 
 		add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
-- 
2.26.2