linux-kernel - [PATCH v2] Fix overflow when HZ is smaller than 60

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Date:	Fri, 24 Jan 2014 16:41:36 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	linux-kernel@...r.kernel.org
Subject: [PATCH v2] Fix overflow when HZ is smaller than 60

The patch that I sent before had wrong numbers in it, it could result in 
"Clocksource jiffies might overflow on 11% adjustment" message. This is 
the patch with correct numbers.

> +#if HZ < 30
> +#define JIFFIES_SHIFT	6
> +#elif HZ < 60
> +#define JIFFIES_SHIFT	7
> +#else
>  #define JIFFIES_SHIFT	8
> +#endif

From: Mikulas Patocka <mpatocka@...hat.com>

Fix overflow when HZ is 32

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:
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@...ax.karlin.mff.cuni.cz>
Cc: stable@...r.kernel.org

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

Index: linux-2.6-ia64/kernel/time/jiffies.c
===================================================================
--- linux-2.6-ia64.orig/kernel/time/jiffies.c	2014-01-24 22:34:17.000000000 +0100
+++ linux-2.6-ia64/kernel/time/jiffies.c	2014-01-24 22:36:56.000000000 +0100
@@ -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
+#else
 #define JIFFIES_SHIFT	8
+#endif
 
 static cycle_t jiffies_read(struct clocksource *cs)
 {
--
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