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

Paul E. McKenney
2010-04-16T14:17:49Z

On Fri, Apr 16, 2010 at 03:51:21PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-04-15 at 07:28 -0700, Paul E. McKenney wrote:
> > On Wed, Apr 14, 2010 at 03:51:50PM +0200, Peter Zijlstra wrote:
> > > On Mon, 2010-04-12 at 20:43 -0700, Paul E. McKenney wrote:
> > > > > So we might have to support the interrupt assumption, at least in some
> > > > > form, with those guys...
> > > >
> > > > One way to make the interrupt assumption official is to use
> > > > synchronize_sched() rather than synchronize_rcu().
> > >
> > > Well, call_rcu_sched() then, because the current usage is to use
> > > call_rcu() to free the page directories.
> > >
> > > Paul, here is a call_rcu_sched() available in kernel/rcutree.c, but am I
> > > right in reading that code that that would not be available for
> > > preemptible RCU?
> >
> > Both call_rcu_sched() and call_rcu() are always there for you. ;-)
> >
> > o If CONFIG_TREE_RCU (or CONFIG_TINY_RCU), they both have the same
> > implementation.
> >
> > o If CONFIG_TREE_PREEMPT_RCU, call_rcu_sched() is preemptible and
> > call_rcu() is not.
>
> (The reverse I suspect?)

Indeed: If CONFIG_TREE_PREEMPT_RCU, call_rcu() is preemptible and
call_rcu_sched() is not.

> > 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?

> > Can the code in question accommodate these restrictions?
>
> Yes, that should do just fine I think.

Cool!!!

Thanx, Paul