[patch] ARM: smpboot: Enable interrupts after marking CPU online/active

[Thomas Gleixner]

On Tue, 15 Nov 2011, Stepan Moskovchenko wrote:
> I am seeing a deadlock when executing hotplug operations with this patch
> applied. When the secondary CPU gets brought up in _cpu_up, the cpu is turned
> on
> and then the online notifier gets called, which is what marks the secondary
> CPU
> as active. If _cpu_up on the primary CPU is preempted before the secondary CPU
> is marked active, it is possible that the primary CPU will want to call
> smp_call_function (or send an IPI) to the secondary CPU because it is marked
> online. However, with this patch, the secondary CPU is still spinning on
> !cpu_active(cpu)
> with interrupts disabled. So, the primary CPU is now stuck in csd_lock_wait(),
> waiting for the secondary CPU to respond, while the secondary CPU spins with
> interrupts disabled, waiting for the primary CPU to mark it as active. So,
> while
> your approach to not call smp_function_single may work for you in your
> specific
> case, I believe there is still a problem in the general case.
> 
> One suggestion for resolving this might be making smp_call_function look at
> the
> active CPUs rather than online CPUs, or to just let the secondary CPU mark
> itself as active rather than having the primary CPU do this, though this might
> defeat the original intended purpose of the active mask.

What a mess. I'll have a look tomorrow.