Re: [PATCH tip/core/rcu 2/3] rcu: Provide polling interfaces for Tiny RCU grace periods

[Paul E. McKenney]

On Sun, Mar 21, 2021 at 11:28:55PM +0100, Frederic Weisbecker wrote:
> On Wed, Mar 03, 2021 at 04:26:31PM -0800, paulmck@xxxxxxxxxx wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > 
> > There is a need for a non-blocking polling interface for RCU grace
> > periods, so this commit supplies start_poll_synchronize_rcu() and
> > poll_state_synchronize_rcu() for this purpose.  Note that the existing
> > get_state_synchronize_rcu() may be used if future grace periods are
> > inevitable (perhaps due to a later call_rcu() invocation).  The new
> > start_poll_synchronize_rcu() is to be used if future grace periods
> > might not otherwise happen.  Finally, poll_state_synchronize_rcu()
> > provides a lockless check for a grace period having elapsed since
> > the corresponding call to either of the get_state_synchronize_rcu()
> > or start_poll_synchronize_rcu().
> > 
> > As with get_state_synchronize_rcu(), the return value from either
> > get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
> > to a later call to either poll_state_synchronize_rcu() or the existing
> > (might_sleep) cond_synchronize_rcu().
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> >  include/linux/rcutiny.h | 11 ++++++-----
> >  kernel/rcu/tiny.c       | 40 ++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 46 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index 2a97334..69108cf4 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -17,14 +17,15 @@
> >  /* Never flag non-existent other CPUs! */
> >  static inline bool rcu_eqs_special_set(int cpu) { return false; }
> >  
> > -static inline unsigned long get_state_synchronize_rcu(void)
> > -{
> > -	return 0;
> > -}
> > +unsigned long get_state_synchronize_rcu(void);
> > +unsigned long start_poll_synchronize_rcu(void);
> > +bool poll_state_synchronize_rcu(unsigned long oldstate);
> >  
> >  static inline void cond_synchronize_rcu(unsigned long oldstate)
> >  {
> > -	might_sleep();
> > +	if (poll_state_synchronize_rcu(oldstate))
> > +		return;
> > +	synchronize_rcu();
> 
> Perhaps cond_synchronize_rcu() could stay as it was. If it might
> call synchronize_rcu() then it inherits its constraint to be
> called from a quiescent state.

As in leave the might_sleep()?  How about something like this?

static inline void cond_synchronize_rcu(unsigned long oldstate)
{
	if (!poll_state_synchronize_rcu(oldstate))
		synchronize_rcu();
	else
		might_sleep();
}

One advantage of this is that the Tiny and Tree implementations
become identical and can then be consolidated.

Or did I miss your point?

						Thanx, Paul