Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

[Peter Zijlstra]

On Wed, Apr 19, 2017 at 08:08:09AM -0700, Paul E. McKenney wrote:
> And even that would not be completely sufficient.  After all, the state
> in the leaf rcu_node structure will be out of date during grace-period
> initialization and cleanup.  So to -completely- synchronize state for
> the incoming CPU, I would have to acquire the root rcu_node structure's
> lock and look at the live state.  Needless to say, the performance and
> scalability implications of acquiring a global lock on each and every
> idle exit event is not going to be at all pretty.

Arguably you could use a seqlock to read the global state. Will still
ponder things a bit more, esp. those bugs you pointed me at from just
reading gpnum.