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

On 02/19/2013 03:10 PM, Michel Lespinasse wrote:

On Tue, Feb 19, 2013 at 2:50 AM, Srivatsa S. Bhat
[off-list ref] wrote:

quoted

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 :-/

quoted

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