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Date:   Thu, 6 Jun 2019 14:17:36 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <Will.Deacon@....com>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: single copy atomicity for double load/stores on 32-bit systems

On Thu, Jun 06, 2019 at 04:34:52PM +0000, David Laight wrote:
> From: Paul E. McKenney
> > Sent: 06 June 2019 10:44
> ...
> > But m68k is !SMP-only, correct?  If so, the only issues would be
> > interactions with interrupt handlers and the like, and doesn't current
> > m68k hardware use exact interrupts?  Or is it still possible to interrupt
> > an m68k in the middle of an instruction like it was in the bad old days?
> 
> Hardware interrupts were always on instruction boundaries, the
> mid-instruction interrupts would only happen for page faults (etc).

OK, !SMP should be fine, then.

> There were SMP m68k systems (but I can't remember one).
> It was important to continue from a mid-instruction trap on the
> same cpu - unless you could guarantee that all the cpus had
> exactly the same version of the microcode.

Yuck!  ;-)

> In any case you could probably use the 'cmp2' instruction
> for an atomic 64bit write.
> OTOH setting that up was such a PITA it was always easier
> to disable interrupts.

Unless I am forgetting something, given that m68k is a 32-bit system,
we should be OK without an atomic 64-bit write.

							Thanx, Paul

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