linux-kernel - [PATCH 1/2] time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()

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Date:	Wed,  9 Sep 2015 16:07:30 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Nuno Gonçalves <nunojpg@...il.com>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, <stable@...r.kernel.org>
Subject: [PATCH 1/2] time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()

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 dc491596f639438 (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.

Cc: Nuno Gonçalves <nunojpg@...il.com>
Cc: Miroslav Lichvar <mlichvar@...hat.com>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: <stable@...r.kernel.org> # v3.17+
Reported-by: Nuno Gonçalves <nunojpg@...il.com>
Tested-by: Nuno Goncalves <nunojpg@...il.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 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 f6ee2e6..3739ac6 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1614,7 +1614,7 @@ 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

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