Re: single copy atomicity for double load/stores on 32-bit systems

["Paul E. McKenney" <paulmck@linux.ibm.com>]

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