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

Peter Zijlstra
2010-04-16T15:14:41Z

On Fri, 2010-04-16 at 08:09 -0700, Paul E. McKenney wrote:
> On Fri, Apr 16, 2010 at 04:56:50PM +0200, Peter Zijlstra wrote:
> > 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.
>
> It is actually not permitted to do general blocking in a preemptible RCU
> read-side critical section. Otherwise, someone is going to block waiting
> for a network packet that never comes, thus OOMing the system.

Sure, something that guarantees progress seems like a sensible
restriction for any lock, and in particular RCU :-)