[PATCH] powerpc: Only obtain cpu_hotplug_lock if called by rtasd

[Michael Ellerman]

Thiago Jung Bauermann <bauerman at linux.vnet.ibm.com> writes:

> Michael Ellerman <mpe at ellerman.id.au> writes:
>> Thiago Jung Bauermann <bauerman at linux.vnet.ibm.com> writes:
>>
>>> Calling arch_update_cpu_topology from a CPU hotplug state machine callback
>>> hits a deadlock because the function tries to get a read lock on
>>> cpu_hotplug_lock while the state machine still holds a write lock on it.
>>>
>>> Since all callers of arch_update_cpu_topology except rtasd already hold
>>> cpu_hotplug_lock, this patch changes the function to use
>>> stop_machine_cpuslocked and creates a separate function for rtasd which
>>> still tries to obtain the lock.
>>>
>>> Michael Bringmann investigated the bug and provided a detailed analysis
>>> of the deadlock on this previous RFC for an alternate solution:
>>>
>>> https://patchwork.ozlabs.org/patch/771293/
>>
>> Do we know when this broke? Or has it never worked?
>
> It's been broken since at least v4.4, I think. I don't know about
> earlier versions.

OK.

Just to be clear, this is happening on a 4.12-rcX system with no other
patches?


The code in arch_update_cpu_topology() has used stop_machine() since 
30c05350c39d ("powerpc/pseries: Use stop machine to update cpu maps")
which went into v3.10, about 4 years ago.

Prior to that it used get/put_online_cpus(), since 9eff1a38407c
("powerpc/pseries: Poll VPA for topology changes and update NUMA maps"),
which was 2.6.38 in 2010.

I wouldn't rule out the possibility it's been broken for 7 years, but I
wonder if something else has changed to cause it to break.

We really need to work it out before we backport anything.

>> Should it go to stable? (can't in its current form AFAICS)
>
> It's not hard to backport both this patch and commit fe5595c07400
> ("stop_machine: Provide stop_machine_cpuslocked()") from branch
> smp/hotplug in tip.git for stable.

Yeah but it's not really my business backporting that unfortunately.

> Since rtasd only started calling arch_update_cpu_topology since v4.11,
> for earlier versions this patch can be simplified to making that
> function call stop_machine_cpuslocked unconditionally instead of
> defining a separate function.

OK.

cheers