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Subject[PATCH] system timer: fix crash in <100Hz system timer
The kernel has a divide by zero crash when trying to run the system
timer less than 100Hz. The problem is x/(HZ/USER_HZ) and related.
Now x*(USER_HZ/HZ) will be used if HZ<USER_HZ.

I'm running the Linux kernel under qemu and went to run a slower
system timer to take less CPU (and battery) on the host. I found that
the kernel paniced under emulation because of a divide by zero in
three places. Here is the patch. The base git was updated today
01-05-2008. I went for a 20Hz system time by adding config HZ_20 etc
to kernel/Kconfig.hz. With this patch I verified the system timer by
looking at /proc/interrupts.

Signed-off-by: David Fries <david@fries.net>

--
David Fries <david@fries.net>
http://fries.net/~david/ (PGP encryption key available)


diff --git a/include/linux/acct.h b/include/linux/acct.h
index 302eb72..86b848d 100644
--- a/include/linux/acct.h
+++ b/include/linux/acct.h
@@ -173,7 +173,11 @@ typedef struct acct acct_t;
static inline u32 jiffies_to_AHZ(unsigned long x)
{
#if (TICK_NSEC % (NSEC_PER_SEC / AHZ)) == 0
- return x / (HZ / AHZ);
+ #if HZ < AHZ
+ return x * (AHZ / HZ);
+ #else
+ return x / (HZ / AHZ);
+ #endif
#else
u64 tmp = (u64)x * TICK_NSEC;
do_div(tmp, (NSEC_PER_SEC / AHZ));
diff --git a/kernel/time.c b/kernel/time.c
index 09d3c45..23af26f 100644
--- a/kernel/time.c
+++ b/kernel/time.c
@@ -565,7 +565,11 @@ EXPORT_SYMBOL(jiffies_to_timeval);
clock_t jiffies_to_clock_t(long x)
{
#if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
+ #if HZ < USER_HZ
+ return x * (USER_HZ / HZ);
+ #else
return x / (HZ / USER_HZ);
+ #endif
#else
u64 tmp = (u64)x * TICK_NSEC;
do_div(tmp, (NSEC_PER_SEC / USER_HZ));
@@ -598,7 +602,12 @@ EXPORT_SYMBOL(clock_t_to_jiffies);
u64 jiffies_64_to_clock_t(u64 x)
{
#if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
- do_div(x, HZ / USER_HZ);
+ #if HZ < USER_HZ
+ x *= USER_HZ;
+ do_div(x, HZ);
+ #else
+ do_div(x, HZ / USER_HZ);
+ #endif
#else
/*
* There are better ways that don't overflow early,[unhandled content-type:application/pgp-signature]
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