Armada XP (MV78460): BUG in netdevice.h with maxcpus=2

[Russell King - ARM Linux]

On Fri, Jan 08, 2016 at 08:45:23PM +0800, Jisheng Zhang wrote:
> let's assume a quad core system, boot with maxcpus=2, after booting.
> 
> on arm64, present cpus is cpu0, cpu1
> 
> on arm, present cpus is cpu0, cpu1, cpu2 and cpu3.
> 
> the arm core code requires every platform to update the present map in
> platforms' smp_prepare_cpus(), but only two or three platforms do so.

The behaviour of maxcpus= is architecture specific.  Some architectures
take notice of this to limit the number of present and possible CPUs,
others ignore it.  In any case, it limits the number of CPUs that come
online at boot.

However, that is a complete red herring, because what we're talking
about is bringing up the network interface at some point later, when
CPU hotplug may have changed the online state already: the kernel may
have booted with CPU0-3 online, but CPUs 2 and 3 may have been taken
offline before the network interface is brought up.

> What's the better solution? Could you please guide me?

I would suggest that you need to track the state of each CPU within
your percpu specific code with appropriate locks to prevent concurrency,
and ensure that you register the CPU hotplug notifier just before
walking the currently on-line CPUs.

Also, walk the list of currently on-line CPUs (using, eg,
smp_call_function) just before unregistering the hotplug notifier.

Remember that your per-CPU bringup/takedown may be executed twice
for the same CPU - once via the hotplug notifier and once via
smp_call_function() - hence why you need locking and state tracking.

That fixes two of the sites.  For the other site, I wonder whether
it's possible to restructure the code so there's no need to have
three sites, since it's fairly obvious when thinking about the
CPU hotplug case, you only have notification of the CPU coming
online and the CPU going away.  It's been a while since I looked
at the mvneta code to really comment on that though.

-- 
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