Patch "time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()" has been added to the 4.2-stable tree

[<gregkh@linuxfoundation.org>]

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

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

to the 4.2-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-timekeeping_freqadjust-s-incorrect-use-of-abs-instead-of-abs64.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 2619d7e9c92d524cb155ec89fd72875321512e5b Mon Sep 17 00:00:00 2001
From: John Stultz <john.stultz@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
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From: John Stultz <john.stultz@linaro.org>

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@gmail.com>
Tested-by: Nuno Goncalves <nunojpg@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1441840051-20244-1-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/time/timekeeping.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1607,7 +1607,7 @@ static __always_inline void timekeeping_
 	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;
 


Patches currently in stable-queue which might be from john.stultz@linaro.org are

queue-4.2/time-fix-timekeeping_freqadjust-s-incorrect-use-of-abs-instead-of-abs64.patch