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

[Paul E. McKenney]

On Mon, Mar 22, 2021 at 08:00:35PM +0100, Frederic Weisbecker wrote:
> On Mon, Mar 22, 2021 at 08:47:44AM -0700, Paul E. McKenney wrote:
> > 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?
> 
> But poll_state_synchronize_rcu() checks that the gp_num has changed,
> which is not needed for cond_synchronize_rcu() since this it is
> only allowed to be called from a QS.

Good catch, and thank you!  Back to a single might_sleep() it is!

						Thanx, Paul