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SubjectRe: [PATCH] system timer: fix crash in <100Hz system timer
On Sat, 5 Jan 2008 16:16:55 -0600
David Fries <david@fries.net> wrote:

> 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,

Alas, I get 100% rejects due to conflicting changes from Peter's
avoid-overflows-in-kernel-timec.patch.

Peter, did that patch propagate this failure, or might it have happily
fixed it?



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