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. > > Thanx, Paul