Subject: Re: [PATCH v6 08/46] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

From: "Srivatsa S. Bhat" <[email-masked]>
Date: Tue, 19 Feb 2013 15:25:47 +0530

On 02/19/2013 03:10 PM, Michel Lespinasse wrote:
> On Tue, Feb 19, 2013 at 2:50 AM, Srivatsa S. Bhat
> <[email-masked]> wrote:
>> But, the whole intention behind removing the parts depending on the
>> recursive property of rwlocks would be to make it easier to make rwlocks
>> fair (going forward) right? Then, that won't work for CPU hotplug, because,
>> just like we have a legitimate reason to have recursive
>> get_online_cpus_atomic(), we also have a legitimate reason to have
>> unfairness in locking (i.e., for deadlock-safety). So we simply can't
>> afford to make the locking fair - we'll end up in too many deadlock
>> possibilities, as hinted in the changelog of patch 1.
> 
> Grumpf - I hadn't realized that making the underlying rwlock fair
> would break your hotplug use case. But you are right, it would. Oh
> well :/
> 

Yeah :-/

>> So the only long-term solution I can think of is to decouple
>> percpu-rwlocks and rwlock_t (like what Tejun suggested) by implementing
>> our own unfair locking scheme inside. What do you think?
> 
> I have no idea how hard would it be to change get_online_cpus_atomic()
> call sites so that the hotplug rwlock read side has a defined order vs
> other locks (thus making sure the situation you describe in patch 1
> doesn't happen). I agree we shouldn't base our short term plans around
> that, but maybe that's doable in the long term ???
>

I think it should be possible in the longer term. I'm expecting it to be
*much much* harder to audit and convert (requiring a lot of subsystem
knowledge of each subsystem that we are touching), than the simpler
tree-wide conversion that I did in this patchset... but I don't think it
is impossible.
 
> Otherwise, I think we should add some big-fat-warning that percpu
> rwlocks don't have reader/writer fairness, that the hotplug use case
> actually depends on the unfairness / would break if the rwlock was
> made fair, and that any new uses of percpu rwlocks should be very
> carefully considered because of the reader/writer fairness issues.

In fact, when I started out, I actually contained all the new locking code
inside CPU hotplug itself, and didn't even expose it as a generic percpu
rwlock in some of the previous versions of this patchset... :-)

But now that we already have a generic locking scheme exposed, we could
add a warning against using it without due consideration.

> Maybe even give percpu rwlocks a less generic sounding name, given how
> constrained they are by the hotplug use case.
 
I wouldn't go that far... ;-) Unfairness is not a show-stopper right?
IMHO, the warning/documentation should suffice for anybody wanting to
try out this locking scheme for other use-cases.

Regards,
Srivatsa S. Bhat