From owner-freebsd-current  Sun Feb 17 16:52:45 2002
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Date: Mon, 18 Feb 2002 11:52:26 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Matthew Dillon <dillon@apollo.backplane.com>
Cc: Poul-Henning Kamp <phk@critter.freebsd.dk>,
	<freebsd-current@FreeBSD.ORG>
Subject: Re: Success!  Sorta!  (was Re: 'microuptime() went backwards ...'
 using ACPI timer. Shouldn't that be impossible? )
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On Sun, 17 Feb 2002, Matthew Dillon wrote:

>     Whoop!  I take it back.  I'm still getting the errors:
>
> microuptime() went backwards (458.168990 -> 458.168882)
> microuptime() went backwards (578.609995 -> 577.929801)
> microuptime() went backwards (748.912755 -> 748.237402)
> microuptime() went backwards (775.159625 -> 775.159612)
>
>     I also think this retry loop has to be done everywhere where the
>     timecounter structure is accessed directly.

Yes, since the reads of all the relevant timecounter variables are
non-atomic.

> Index: kern/kern_tc.c
> ===================================================================
> RCS file: /home/ncvs/src/sys/kern/kern_tc.c,v
> retrieving revision 1.113
> diff -u -r1.113 kern_tc.c
> --- kern/kern_tc.c	7 Feb 2002 21:21:55 -0000	1.113
> +++ kern/kern_tc.c	17 Feb 2002 20:41:47 -0000
> @@ -126,8 +128,10 @@
>  	struct timecounter *tc;
>
>  	ngetmicrotime++;
> -	tc = timecounter;
> -	*tvp = tc->tc_microtime;
> +	do {
> +		tc = timecounter;
> +		*tvp = tc->tc_microtime;
> +	} while (tc != timecounter);
>  }
>
>  void

E.g., tc_mictrotime here is a timeval.  It doesn't matter getting a stale
value (although getting a stale value increases the possible incoherency
of the get*() functions from 1/HZ to NTIMECOUNTER/HZ), but getting a
stale value that changed underneath the read would be bad.

Bruce


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