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

Paul E. McKenney
2010-04-15T14:29:05Z

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.

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.

Can the code in question accommodate these restrictions?

Thanx, Paul