Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2010-04-16 at 07:32 -0700, Paul E. McKenney wrote:
> On Fri, Apr 16, 2010 at 04:23:39PM +0200, Peter Zijlstra wrote:
> > On Fri, 2010-04-16 at 07:17 -0700, Paul E. McKenney wrote:
> > 
> > > > > Of course, with call_rcu_sched(), the corresponding RCU read-side critical
> > > > > sections are non-preemptible.  Therefore, in CONFIG_PREEMPT_RT, these
> > > > > read-side critical sections must use raw spinlocks.
> > > > 
> > > > OK, so if we fully remove CONFIG_TREE_PREEMPT_RCU (defaulting to y),
> > > > rename all the {call_rcu, rcu_read_lock, rcu_read_unlock,
> > > > synchronize_rcu} functions to {*}_preempt and then add a new
> > > > CONFIG_PREEMPT_RCU that simply maps {*} to either {*}_sched or
> > > > {*}_preempt, we've basically got what I've been asking for for a while,
> > > > no?
> > > 
> > > What would rcu_read_lock_preempt() do in a !CONFIG_PREEMPT kernel?
> > 
> > Same as for a preempt one, since you'd have to be able to schedule()
> > while holding it to be able to do things like mutex_lock().
> 
> So what you really want is something like rcu_read_lock_sleep() rather
> than rcu_read_lock_preempt(), right?  The point is that you want to do
> more than merely preempt, given that it is legal to do general blocking
> while holding a mutex, correct?

Right, but CONFIG_TREE_PREEMPT_RCU=y ends up being that. We could change
the name to _sleep, but we've been calling it preemptible-rcu for a long
while now.