linux-kernel - [PATCH 07/12] time: Try to catch clocksource delta underflows

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Message-Id: <1425696603-16878-8-git-send-email-john.stultz@linaro.org>
Date:	Fri,  6 Mar 2015 18:49:58 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	lkml <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 07/12] time: Try to catch clocksource delta underflows

In the case where there is a broken clocksource
where there are multiple actual clocks that
aren't perfectly aligned, we may see small "negative"
deltas when we subtract now from cycle_last.

The values are actually negative with respect to the
clocksource mask value, not necessarily negative
if cast to a s64, but we can check by checking the
delta see if it is a small (relative to the mask)
negative value (again negative relative to the mask).

If so, we assume we jumped backwards somehow and
instead use zero for our delta.

Cc: Dave Jones <davej@...emonkey.org.uk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Stephen Boyd <sboyd@...eaurora.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 kernel/time/timekeeping.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 8b9e328..4e8ccde 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -145,6 +145,13 @@ static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr)
 	/* calculate the delta since the last update_wall_time */
 	delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask);
 
+	/*
+	 * Try to catch underflows by checking if we are seeing small
+	 * mask-relative negative values.
+	 */
+	if (unlikely((~delta & tkr->mask) < (tkr->mask >> 3)))
+		delta = 0;
+
 	/* Cap delta value to the max_cycles values to avoid mult overflows */
 	if (unlikely(delta > tkr->clock->max_cycles))
 		delta = tkr->clock->max_cycles;
-- 
1.9.1

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