clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin

Right now, cs_watchdog_read() does clocksource sanity checks based
on WATCHDOG_MAX_SKEW, which sets a floor on any clocksource's
.uncertainty_margin.  These sanity checks can therefore act
inappropriately for clocksources with large uncertainty margins.

One reason for a clocksource to have a large .uncertainty_margin is when
that clocksource has long read-out latency, given that it does not make
sense for the .uncertainty_margin to be smaller than the read-out latency.
With the current checks, cs_watchdog_read() could reject all normal
reads from a clocksource with long read-out latencies, such as those
from legacy clocksources that are no longer implemented in hardware.

Therefore, recast the cs_watchdog_read() checks in terms of the
.uncertainty_margin values of the clocksources involved in the timespan in
question.  The first covers two watchdog reads and one cs read, so use
twice the watchdog .uncertainty_margin plus that of the cs.  The second
covers only a pair of watchdog reads, so use twice the watchdog
.uncertainty_margin.

Reported-by: Borislav Petkov <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

Author: paulmckrcu

kernel/time/clocksource.c

@@ -244,6 +244,7 @@ enum wd_read_status {
 
 static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
 {
+	int64_t md = 2 * watchdog->uncertainty_margin;
 	unsigned int nretries, max_retries;
 	int64_t wd_delay, wd_seq_delay;
 	u64 wd_end, wd_end2;
@@ -258,7 +259,7 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
 		local_irq_enable();
 
 		wd_delay = cycles_to_nsec_safe(watchdog, *wdnow, wd_end);
-		if (wd_delay <= WATCHDOG_MAX_SKEW) {
+		if (wd_delay <= md + cs->uncertainty_margin) {
 			if (nretries > 1 && nretries >= max_retries) {
 				pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n",
 					smp_processor_id(), watchdog->name, nretries);
@@ -271,12 +272,12 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
 		 * there is too much external interferences that cause
 		 * significant delay in reading both clocksource and watchdog.
 		 *
-		 * If consecutive WD read-back delay > WATCHDOG_MAX_SKEW/2,
-		 * report system busy, reinit the watchdog and skip the current
+		 * If consecutive WD read-back delay > md, report
+		 * system busy, reinit the watchdog and skip the current
 		 * watchdog test.
 		 */
 		wd_seq_delay = cycles_to_nsec_safe(watchdog, wd_end, wd_end2);
-		if (wd_seq_delay > WATCHDOG_MAX_SKEW/2)
+		if (wd_seq_delay > md)
 			goto skip_test;
 	}