[tip:timers/urgent] time: Fix overflow when HZ is smaller than 60

From: tip-bot for Mikulas Patocka
Date: Thu Feb 06 2014 - 10:04:56 EST

Commit-ID: 80d767d770fd9c697e434fd080c2db7b5c60c6dd
Gitweb: http://git.kernel.org/tip/80d767d770fd9c697e434fd080c2db7b5c60c6dd
Author: Mikulas Patocka <mpatocka@xxxxxxxxxx>
AuthorDate: Fri, 24 Jan 2014 16:41:36 -0500
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 6 Feb 2014 16:01:40 +0100

time: Fix overflow when HZ is smaller than 60

When compiling for the IA-64 ski emulator, HZ is set to 32 because the
emulation is slow and we don't want to waste too many cycles processing
timers. Alpha also has an option to set HZ to 32.

This causes integer underflow in
kernel/time/jiffies.c:66:2: warning: large integer implicitly truncated to unsigned type [-Woverflow]
.mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */

This patch reduces the JIFFIES_SHIFT value to avoid the overflow.

Signed-off-by: Mikulas Patocka <mikulas@xxxxxxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/alpine.LRH.2.02.1401241639100.23871@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

kernel/time/jiffies.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index 7a925ba..a6a5bf5 100644
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -51,7 +51,13 @@
* HZ shrinks, so values greater than 8 overflow 32bits when
* HZ=100.
+#if HZ < 34
+#define JIFFIES_SHIFT 6
+#elif HZ < 67
+#define JIFFIES_SHIFT 7

static cycle_t jiffies_read(struct clocksource *cs)
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