linux-kernel - [PATCH 1/2] time: Fix a bug in timekeeping

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Date:	Thu, 19 Mar 2015 10:42:31 +0800
From:	Xunlei Pang <xlpang@....com>
To:	linux-kernel@...r.kernel.org
Cc:	rtc-linux@...glegroups.com, Thomas Gleixner <tglx@...utronix.de>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	John Stultz <john.stultz@...aro.org>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: [PATCH 1/2] time: Fix a bug in timekeeping_suspend() with no persistent clock

From: Xunlei Pang <pang.xunlei@...aro.org>

When there's no persistent clock, normally timekeeping_suspend_time
should always be zero, but this can break in timekeeping_suspend().

At T1, there was a system suspend, so old_delta was assigned T1.
After some time, one time adjustment happened, and xtime got the
value of T1-dt(0s<dt<2s). Then, there comes another system suspend
soon after this adjustment, obviously we will get a small negative
delta_delta, resulting in a negative timekeeping_suspend_time.

This is problematic, when doing timekeeping_resume() if there is
no nonstop clocksource for example, it will hit the else leg and
inject the improper sleeptime which is the wrong logic.

So, we can solve this problem by only doing delta related code when
the persistent clock is existent. Actually the code only makes sense
for persistent clock cases.

Signed-off-by: Xunlei Pang <pang.xunlei@...aro.org>
---
 kernel/time/timekeeping.c | 36 +++++++++++++++++++-----------------
 1 file changed, 19 insertions(+), 17 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 1345e63..605da5c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1127,7 +1127,7 @@ void __init timekeeping_init(void)
 	raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
 }
 
-/* time in seconds when suspend began */
+/* time in seconds when suspend began for persistent clock */
 static struct timespec64 timekeeping_suspend_time;
 
 /**
@@ -1304,24 +1304,26 @@ int timekeeping_suspend(void)
 	timekeeping_forward_now(tk);
 	timekeeping_suspended = 1;
 
-	/*
-	 * To avoid drift caused by repeated suspend/resumes,
-	 * which each can add ~1 second drift error,
-	 * try to compensate so the difference in system time
-	 * and persistent_clock time stays close to constant.
-	 */
-	delta = timespec64_sub(tk_xtime(tk), timekeeping_suspend_time);
-	delta_delta = timespec64_sub(delta, old_delta);
-	if (abs(delta_delta.tv_sec)  >= 2) {
+	if (has_persistent_clock()) {
 		/*
-		 * if delta_delta is too large, assume time correction
-		 * has occured and set old_delta to the current delta.
+		 * To avoid drift caused by repeated suspend/resumes,
+		 * which each can add ~1 second drift error,
+		 * try to compensate so the difference in system time
+		 * and persistent_clock time stays close to constant.
 		 */
-		old_delta = delta;
-	} else {
-		/* Otherwise try to adjust old_system to compensate */
-		timekeeping_suspend_time =
-			timespec64_add(timekeeping_suspend_time, delta_delta);
+		delta = timespec64_sub(tk_xtime(tk), timekeeping_suspend_time);
+		delta_delta = timespec64_sub(delta, old_delta);
+		if (abs(delta_delta.tv_sec) >= 2) {
+			/*
+			 * if delta_delta is too large, assume time correction
+			 * has occurred and set old_delta to the current delta.
+			 */
+			old_delta = delta;
+		} else {
+			/* Otherwise try to adjust old_system to compensate */
+			timekeeping_suspend_time =
+				timespec64_add(timekeeping_suspend_time, delta_delta);
+		}
 	}
 
 	timekeeping_update(tk, TK_MIRROR);
-- 
1.9.1


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