Re: [Patch] Re: Nasty suprise with uptime

[Andreas Dilger]

On Oct 31, 2001  21:47 +0100, Tim Schmielau wrote:
> OK, I introduced get_jiffies64, corrected my 64 bit mistake and
> subtract INITIAL_JIFFIES to obtain uptime, while leaving it at at pre-wrap
> value for error-chasing.
> Still this patch introduces jiffies_high on 64 bit platforms which will be
> useless until the year 571234830.

It probably won't be accepted until the whole thing disappears for 64-bit
systems.

> btw.: can someone please explain to me why do_timer uses
> 	(*(unsigned long *)&jiffies)++;
> instead of just doing jiffies++ ?

Can't say for sure, but it may be a compiler issue, maybe historical.

>  	 */
>  #if HZ!=100
>  	len = sprintf(page,"%lu.%02lu %lu.%02lu\n",
> -		uptime / HZ,
> -		(((uptime % HZ) * 100) / HZ) % 100,
> +		(unsigned long) uptime,
> +		((remainder * 100) / HZ) % 100,
>  		idle / HZ,
>  		(((idle % HZ) * 100) / HZ) % 100);

Given that uptime is now 64-bits, do we need this mathematical hoop jumping?
Also, won't the remainder already be modulus HZ in this case?

>  void do_timer(struct pt_regs *regs)
>  {
> -	(*(unsigned long *)&jiffies)++;
> +	/* we assume that two calls to do_timer can never overlap
> +	 * since they are one jiffie apart in time */
> +	if (jiffies != (unsigned long)(-1)) {
> +		jiffies++;
> +	} else {
> +		/* We still need to care about the race with readers of
> +		 * jiffies_hi. Readers have to discard the values if
> +		 * jiffies_hi != jiffies_hi_shadow when read with
> +		 * proper barriers in between. */
> +		jiffies_hi++;
> +		barrier();
> +		jiffies++;
> +		barrier();
> +		jiffies_hi_shadow = jiffies_hi;
> +		barrier();
> +	}

I think this is the part of the patch that people object to.  See my
other posting of how to handle 64-bit jiffies with only an impact to
users of the 64-bit value.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/