Re: [PATCH RFC tip/core/rcu 19/28] nohz: Allow rcu extended quiescent state handling seperately from tick stop

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Nov 02, 2011 at 01:30:40PM -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@gmail.com>
> 
> It is assumed that rcu won't be used once we switch to tickless
> mode and until we restart the tick. However this is not always
> true, as in x86-64 where we dereference the idle notifiers after
> the tick is stopped.
> 
> To prepare for fixing this, add two new APIs:
> tick_nohz_idle_enter_norcu() and tick_nohz_idle_exit_norcu().
> 
> If no use of RCU is made in the idle loop between
> tick_nohz_enter_idle() and tick_nohz_exit_idle() calls, the arch
> must instead call the new *_norcu() version such that the arch doesn't
> need to call rcu_idle_enter() and rcu_idle_exit().

The _norcu names confused me a bit.  At first, I thought they meant
"idle but not RCU idle, so you can use RCU", but from re-reading the
commit message, apparently they mean "idle and RCU idle, so don't use
RCU".  What about something like _forbid_rcu instead?  Or,
alternatively, why not just go ahead and separate the two types of idle
entirely rather than introducing the _norcu variants first?

- Josh Triplett