Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Aug 28, 2014 at 12:47:45PM -0700, Paul E. McKenney wrote:
> Currently, the expedited grace-period primitives do get_online_cpus().
> This greatly simplifies their implementation, but means that calls to
> them holding locks that are acquired by CPU-hotplug notifiers (to say
> nothing of calls to these primitives from CPU-hotplug notifiers) can
> deadlock.  But this is starting to become inconvenient:
> https://lkml.org/lkml/2014/8/5/754

Please recap the actual problem; the link might die and the actual mail
linked to isn't very useful in any case.

> This commit avoids the deadlock and retains the simplicity by creating
> a try_get_online_cpus(), which returns false if the get_online_cpus()
> reference count could not immediately be incremented.  If a call to
> try_get_online_cpus() returns true, the expedited primitives operate
> as before.  If a call returns false, the expedited primitives fall back
> to normal grace-period operations.  This falling back of course results
> in increased grace-period latency, but only during times when CPU
> hotplug operations are actually in flight.  The effect should therefore
> be negligible during normal operation.

URGH.. I really hate that. The hotplug interface is already too
horrible, we should not add such hacks to it.

How about ripping that rcu_expedited stuff out instead? That's all
conditional anyhow, so might as well not do it.