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Message-ID: <17417.35072.247188.486774@cargo.ozlabs.ibm.com>
Date: Sat, 4 Mar 2006 23:33:04 +1100
From: Paul Mackerras <paulus@samba.org>
To: Andrew Morton <akpm@osdl.org>
Cc: ak@muc.de, anemo@mba.ocn.ne.jp, clameter@engr.sgi.com,
       linux-kernel@vger.kernel.org, ralf@linux-mips.org, johnstul@us.ibm.com,
       rth@twiddle.net
Subject: Re: [PATCH] simplify update_times (avoid jiffies/jiffies_64
 aliasing problem)
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Andrew Morton writes:
> Paul Mackerras <paulus@samba.org> wrote:
> >
> > Andi Kleen writes:
> > 
> > > Also I assume Atsushi-san did the patch because he saw a real problem?
> > 
> > Yes, one which I also saw on PPC.  The compiler (gcc-4) emits loads
> > for jiffies, jiffies64 and wall_jiffies before storing the incremented
> > jiffies64 value back.
> > 
> 
> What was the effect of that?

The effect is that the first call to do_timer doesn't increment xtime.
This explains why the code I have to detect disagreements between
xtime and the time of day as computed from the timebase register was
finding a disagreement on the first tick, which I was scratching my
head over.

There may be other effects on architectures which use wall_jiffies to
detect lost timer ticks.  We don't have that problem on PPC and we
don't use wall_jiffies in computing time of day.

Paul.

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