Skip to contentSkip to conversation

Patch "time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting" has been added to the 4.9-stable tree

PATCH [email protected] 1 participant 1 message Jun 26, 2017

Subsystems (2)
Stable branch Timekeeping, clocksource core, NTP, alarmtimer
lore
Share & tools
lore

[email protected]

author maintainer Applied
#0 lore raw

This is a note to let you know that I've just added the patch titled

time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
time-fix-clock_monotonic_raw-sub-nanosecond-accounting.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 3d88d56c5873f6eebe23e05c3da701960146b801 Mon Sep 17 00:00:00 2001

From: John Stultz <[email protected]>
Date: Thu, 8 Jun 2017 16:44:21 -0700
Subject: time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting

From: John Stultz <[email protected]>

commit 3d88d56c5873f6eebe23e05c3da701960146b801 upstream.

Due to how the MONOTONIC_RAW accumulation logic was handled, there is the potential for a 1ns discontinuity when we do accumulations. This small discontinuity has for the most part gone un-noticed, but since ARM64 enabled CLOCK_MONOTONIC_RAW in their vDSO clock_gettime implementation, we've seen failures with the inconsistency-check test in kselftest.

This patch addresses the issue by using the same sub-ns accumulation handling that CLOCK_MONOTONIC uses, which avoids the issue for in-kernel users.

Since the ARM64 vDSO implementation has its own clock_gettime calculation logic, this patch reduces the frequency of errors, but failures are still seen. The ARM64 vDSO will need to be updated to include the sub-nanosecond xtime_nsec values in its calculation for this issue to be completely fixed.

Signed-off-by John Stultz <[email protected]>
Tested-by Daniel Mentz <[email protected]>
Cc Prarit Bhargava <[email protected]>
Cc Kevin Brodsky <[email protected]>
Cc Richard Cochran <[email protected]>
Cc Stephen Boyd <[email protected]>
Cc Will Deacon <[email protected]>
Cc Miroslav Lichvar <[email protected]>
Link http://lkml.kernel.org/r/[email protected] open here
Signed-off-by Thomas Gleixner <[email protected]>
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 include/linux/timekeeper_internal.h |    4 ++--
 kernel/time/timekeeping.c           |   20 ++++++++++----------
 2 files changed, 12 insertions(+), 12 deletions(-)
include/linux/timekeeper_internal.h +2 -2
@@ -57,7 +57,7 @@ struct tk_read_base {
  *			interval.
  * @xtime_remainder:	Shifted nano seconds left over when rounding
  *			@cycle_interval
- * @raw_interval:	Raw nano seconds accumulated per NTP interval.
+ * @raw_interval:	Shifted raw nano seconds accumulated per NTP interval.
  * @ntp_error:		Difference between accumulated time and NTP time in ntp
  *			shifted nano seconds.
  * @ntp_error_shift:	Shift conversion between clock shifted nano seconds and

@@ -99,7 +99,7 @@ struct timekeeper {
 	cycle_t			cycle_interval;
 	u64			xtime_interval;
 	s64			xtime_remainder;
-	u32			raw_interval;
+	u64			raw_interval;
 	/* The ntp_tick_length() value currently being used.
 	 * This cached copy ensures we consistently apply the tick
 	 * length for an entire tick, as ntp_tick_length may change
kernel/time/timekeeping.c +10 -10
@@ -278,8 +278,7 @@ static void tk_setup_internals(struct ti
 	/* Go back from cycles -> shifted ns */
 	tk->xtime_interval = (u64) interval * clock->mult;
 	tk->xtime_remainder = ntpinterval - tk->xtime_interval;
-	tk->raw_interval =
-		((u64) interval * clock->mult) >> clock->shift;
+	tk->raw_interval = interval * clock->mult;
 
 	 /* if changing clocks, convert xtime_nsec shift units */
 	if (old_clock) {

@@ -2023,7 +2022,7 @@ static cycle_t logarithmic_accumulation(
 						unsigned int *clock_set)
 {
 	cycle_t interval = tk->cycle_interval << shift;
-	u64 raw_nsecs;
+	u64 snsec_per_sec;
 
 	/* If the offset is smaller than a shifted interval, do nothing */
 	if (offset < interval)

@@ -2038,14 +2037,15 @@ static cycle_t logarithmic_accumulation(
 	*clock_set |= accumulate_nsecs_to_secs(tk);
 
 	/* Accumulate raw time */
-	raw_nsecs = (u64)tk->raw_interval << shift;
-	raw_nsecs += tk->raw_time.tv_nsec;
-	if (raw_nsecs >= NSEC_PER_SEC) {
-		u64 raw_secs = raw_nsecs;
-		raw_nsecs = do_div(raw_secs, NSEC_PER_SEC);
-		tk->raw_time.tv_sec += raw_secs;
+	tk->tkr_raw.xtime_nsec += (u64)tk->raw_time.tv_nsec << tk->tkr_raw.shift;
+	tk->tkr_raw.xtime_nsec += tk->raw_interval << shift;
+	snsec_per_sec = (u64)NSEC_PER_SEC << tk->tkr_raw.shift;
+	while (tk->tkr_raw.xtime_nsec >= snsec_per_sec) {
+		tk->tkr_raw.xtime_nsec -= snsec_per_sec;
+		tk->raw_time.tv_sec++;
 	}
-	tk->raw_time.tv_nsec = raw_nsecs;
+	tk->raw_time.tv_nsec = tk->tkr_raw.xtime_nsec >> tk->tkr_raw.shift;
+	tk->tkr_raw.xtime_nsec -= (u64)tk->raw_time.tv_nsec << tk->tkr_raw.shift;
 
 	/* Accumulate error between NTP and clock interval */
 	tk->ntp_error += tk->ntp_tick << shift;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/time-fix-clock-read-clock-race-around-clocksource-changes.patch queue-4.9/time-fix-clock_monotonic_raw-sub-nanosecond-accounting.patch