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

On Mon, Feb 18, 2013 at 8:39 PM, Srivatsa S. Bhat
[off-list ref] wrote:

Some important design requirements and considerations:
-----------------------------------------------------

1. Scalable synchronization at the reader-side, especially in the fast-path

   Any synchronization at the atomic hotplug readers side must be highly
   scalable - avoid global single-holder locks/counters etc. Because, these
   paths currently use the extremely fast preempt_disable(); our replacement
   to preempt_disable() should not become ridiculously costly and also should
   not serialize the readers among themselves needlessly.

   At a minimum, the new APIs must be extremely fast at the reader side
   atleast in the fast-path, when no CPU offline writers are active.

2. preempt_disable() was recursive. The replacement should also be recursive.

3. No (new) lock-ordering restrictions

   preempt_disable() was super-flexible. It didn't impose any ordering
   restrictions or rules for nesting. Our replacement should also be equally
   flexible and usable.

4. No deadlock possibilities

   Regular per-cpu locking is not the way to go if we want to have relaxed
   rules for lock-ordering. Because, we can end up in circular-locking
   dependencies as explained in https://lkml.org/lkml/2012/12/6/290

   So, avoid the usual per-cpu locking schemes (per-cpu locks/per-cpu atomic
   counters with spin-on-contention etc) as much as possible, to avoid
   numerous deadlock possibilities from creeping in.


Implementation of the design:
----------------------------

We use per-CPU reader-writer locks for synchronization because:

  a. They are quite fast and scalable in the fast-path (when no writers are
     active), since they use fast per-cpu counters in those paths.

  b. They are recursive at the reader side.

  c. They provide a good amount of safety against deadlocks; they don't
     spring new deadlock possibilities on us from out of nowhere. As a
     result, they have relaxed locking rules and are quite flexible, and
     thus are best suited for replacing usages of preempt_disable() or
     local_irq_disable() at the reader side.

Together, these satisfy all the requirements mentioned above.

Thanks for this detailed design explanation.

So, you made it clear why you want a recursive read side here.

I am wondering though, if you could take care of recursive uses in
get/put_online_cpus_atomic() instead of doing it as a property of your
rwlock:

get_online_cpus_atomic()
{
    unsigned long flags;
    local_irq_save(flags);
    if (this_cpu_inc_return(hotplug_recusion_count) == 1)
        percpu_read_lock_irqsafe(&hotplug_pcpu_rwlock);
    local_irq_restore(flags);
}

Once again, the idea there is to avoid baking the reader side
recursive properties into your rwlock itself, so that it won't be
impossible to implement reader/writer fairness into your rwlock in the
future (which may be not be very important for the hotplug use, but
could be when other uses get introduced).

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.