[tip:timers/core] time: Fix a bug in timekeeping_suspend() with no persistent clock

[tip-bot for Xunlei Pang <tipbot@zytor.com>]

Commit-ID:  264bb3f79f2a465477cdcd2f0554e21aedc443a3
Gitweb:     http://git.kernel.org/tip/264bb3f79f2a465477cdcd2f0554e21aedc443a3
Author:     Xunlei Pang <pang.xunlei@linaro.org>
AuthorDate: Wed, 1 Apr 2015 20:34:37 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 3 Apr 2015 08:18:33 +0200

time: Fix a bug in timekeeping_suspend() with no persistent clock

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@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1427945681-29972-18-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.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 3be559b..b7db491 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1255,7 +1255,7 @@ 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;
 
 /**
@@ -1428,24 +1428,26 @@ 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);