watchdog: cleanup handling of false positives

Commit d6ad3e286d ("softlockup: Add sched_clock_tick() to avoid kernel
warning on kgdb resume") introduced touch_softlockup_watchdog_sync().

It solved a problem when the watchdog was touched in an atomic context,
the timer callback was proceed right after releasing interrupts, and the
local clock has not been updated yet.  In this case, sched_clock_tick()
was called in watchdog_timer_fn() before updating the timer.

So far so good.

Later commit 5d1c0f4a80 ("watchdog: add check for suspended vm in
softlockup detector") added two kvm_check_and_clear_guest_paused()
calls.  They touch the watchdog when the guest has been sleeping.

The code makes my head spin around.

Scenario 1:

    + guest did sleep:
	+ PVCLOCK_GUEST_STOPPED is set

    + 1st watchdog_timer_fn() invocation:
	+ the watchdog is not touched yet
	+ is_softlockup() returns too big delay
	+ kvm_check_and_clear_guest_paused():
	   + clear PVCLOCK_GUEST_STOPPED
	   + call touch_softlockup_watchdog_sync()
		+ set SOFTLOCKUP_DELAY_REPORT
		+ set softlockup_touch_sync
	+ return from the timer callback

      + 2nd watchdog_timer_fn() invocation:

	+ call sched_clock_tick() even though it is not needed.
	  The timer callback was invoked again only because the clock
	  has already been updated in the meantime.

	+ call kvm_check_and_clear_guest_paused() that does nothing
	  because PVCLOCK_GUEST_STOPPED has been cleared already.

	+ call update_report_ts() and return. This is fine. Except
	  that sched_clock_tick() might allow to set it already
	  during the 1st invocation.

Scenario 2:

	+ guest did sleep

	+ 1st watchdog_timer_fn() invocation
	    + same as in 1st scenario

	+ guest did sleep again:
	    + set PVCLOCK_GUEST_STOPPED again

	+ 2nd watchdog_timer_fn() invocation
	    + SOFTLOCKUP_DELAY_REPORT is set from 1st invocation
	    + call sched_clock_tick()
	    + call kvm_check_and_clear_guest_paused()
		+ clear PVCLOCK_GUEST_STOPPED
		+ call touch_softlockup_watchdog_sync()
		    + set SOFTLOCKUP_DELAY_REPORT
		    + set softlockup_touch_sync
	    + call update_report_ts() (set real timestamp immediately)
	    + return from the timer callback

	+ 3rd watchdog_timer_fn() invocation
	    + timestamp is set from 2nd invocation
	    + softlockup_touch_sync is set but not checked because
	      the real timestamp is already set

Make the code more straightforward:

1. Always call kvm_check_and_clear_guest_paused() at the very
   beginning to handle PVCLOCK_GUEST_STOPPED. It touches the watchdog
   when the quest did sleep.

2. Handle the situation when the watchdog has been touched
   (SOFTLOCKUP_DELAY_REPORT is set).

   Call sched_clock_tick() when touch_*sync() variant was used. It makes
   sure that the timestamp will be up to date even when it has been
   touched in atomic context or quest did sleep.

As a result, kvm_check_and_clear_guest_paused() is called on a single
location.  And the right timestamp is always set when returning from the
timer callback.

Link: https://lkml.kernel.org/r/20210311122130.6788-7-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
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>

kernel/watchdog.c

@@ -376,7 +376,14 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	/* .. and repeat */
 	hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));
 
-	/* Reset the interval when touched externally by a known slow code. */
+	/*
+	 * If a virtual machine is stopped by the host it can look to
+	 * the watchdog like a soft lockup. Check to see if the host
+	 * stopped the vm before we process the timestamps.
+	 */
+	kvm_check_and_clear_guest_paused();
+
+	/* Reset the interval when touched by known problematic code. */
 	if (period_ts == SOFTLOCKUP_DELAY_REPORT) {
 		if (unlikely(__this_cpu_read(softlockup_touch_sync))) {
 			/*
@@ -387,10 +394,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 			sched_clock_tick();
 		}
 
-		/* Clear the guest paused flag on watchdog reset */
-		kvm_check_and_clear_guest_paused();
 		update_report_ts();
-
 		return HRTIMER_RESTART;
 	}
 
@@ -402,14 +406,6 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	 */
 	duration = is_softlockup(touch_ts, period_ts);
 	if (unlikely(duration)) {
-		/*
-		 * If a virtual machine is stopped by the host it can look to
-		 * the watchdog like a soft lockup, check to see if the host
-		 * stopped the vm before we issue the warning
-		 */
-		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.