[3.16.y-ckt stable] Patch "time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.19.8-ckt9.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From cfd504fea0bee4c977897560c929751a05184f71 Mon Sep 17 00:00:00 2001
From: John Stultz <john.stultz at linaro.org>
Date: Wed, 9 Sep 2015 16:07:30 -0700
Subject: time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead
 of abs64()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit 2619d7e9c92d524cb155ec89fd72875321512e5b upstream.

The internal clocksteering done for fine-grained error
correction uses a logarithmic approximation, so any time
adjtimex() adjusts the clock steering, timekeeping_freqadjust()
quickly approximates the correct clock frequency over a series
of ticks.

Unfortunately, the logic in timekeeping_freqadjust(), introduced
in commit:

  dc491596f639 ("timekeeping: Rework frequency adjustments to work better w/ nohz")

used the abs() function with a s64 error value to calculate the
size of the approximated adjustment to be made.

Per include/linux/kernel.h:

  "abs() should not be used for 64-bit types (s64, u64, long long) - use abs64()".

Thus on 32-bit platforms, this resulted in the clocksteering to
take a quite dampended random walk trying to converge on the
proper frequency, which caused the adjustments to be made much
slower then intended (most easily observed when large
adjustments are made).

This patch fixes the issue by using abs64() instead.

Reported-by: Nuno Gonçalves <nunojpg at gmail.com>
Tested-by: Nuno Goncalves <nunojpg at gmail.com>
Signed-off-by: John Stultz <john.stultz at linaro.org>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Miroslav Lichvar <mlichvar at redhat.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Prarit Bhargava <prarit at redhat.com>
Cc: Richard Cochran <richardcochran at gmail.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Link: http://lkml.kernel.org/r/1441840051-20244-1-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/time/timekeeping.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 6a93185..6e80711 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1431,7 +1431,7 @@ static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
 	negative = (tick_error < 0);

 	/* Sort out the magnitude of the correction */
-	tick_error = abs(tick_error);
+	tick_error = abs64(tick_error);
 	for (adj = 0; tick_error > interval; adj++)
 		tick_error >>= 1;

--
1.9.1