linux-kernel - Re: [PATCH] system timer: fix crash in <100Hz system timer

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Date:	Mon, 07 Jan 2008 15:51:09 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	David Fries <david@...es.net>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] system timer: fix crash in <100Hz system timer

Andrew Morton wrote:
>>
>> 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?
> 

My patch doesn't touch any of these functions, nor touches any code 
within 70 lines of this patch -- the last line touched is line 478 -- 
and doesn't touch linux/acct.h at all, so how could it cause a conflict?

But no, it doesn't fix this particular problem, even if using a similar 
technique might very well be a better way to do this kind of conversion.

	-hpa
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