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Date: Tue, 16 Mar 2021 09:42:29 -0700 From: "Paul E. McKenney" <paulmck@...nel.org> To: Frederic Weisbecker <frederic@...nel.org> Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for Tree RCU grace periods On Tue, Mar 16, 2021 at 03:47:22PM +0100, Frederic Weisbecker wrote: > On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@...nel.org wrote: > > +/** > > + * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period > > + * > > + * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu() > > + * > > + * If a full RCU grace period has elapsed since the earlier call from > > + * which oldstate was obtained, return @true, otherwise return @false. > > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period. > > + * > > + * Yes, this function does not take counter wrap into account. > > + * But counter wrap is harmless. If the counter wraps, we have waited for > > + * more than 2 billion grace periods (and way more on a 64-bit system!). > > + * Those needing to keep oldstate values for very long time periods > > + * (many hours even on 32-bit systems) should check them occasionally > > + * and either refresh them or set a flag indicating that the grace period > > + * has completed. > > + */ > > +bool poll_state_synchronize_rcu(unsigned long oldstate) > > +{ > > + if (rcu_seq_done(&rcu_state.gp_seq, oldstate)) { > > + smp_mb(); /* Ensure GP ends before subsequent accesses. */ > > + return true; > > + } > > + return false; > > +} > > +EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu); > > + > > +/** > > * cond_synchronize_rcu - Conditionally wait for an RCU grace period > > * > > * @oldstate: return value from earlier call to get_state_synchronize_rcu() > > * > > * If a full RCU grace period has elapsed since the earlier call to > > - * get_state_synchronize_rcu(), just return. Otherwise, invoke > > - * synchronize_rcu() to wait for a full grace period. > > + * get_state_synchronize_rcu() or start_poll_synchronize_rcu(), just return. > > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period. > > * > > * Yes, this function does not take counter wrap into account. But > > * counter wrap is harmless. If the counter wraps, we have waited for > > @@ -3804,7 +3864,7 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu); > > */ > > void cond_synchronize_rcu(unsigned long oldstate) > > { > > - if (!rcu_seq_done(&rcu_state.gp_seq, oldstate)) > > + if (!poll_state_synchronize_rcu(oldstate)) > > synchronize_rcu(); > > else > > smp_mb(); /* Ensure GP ends before subsequent accesses. */ > > poll_state_synchronize_rcu() already does the full barrier. Right you are! Good catch, thank you!!! I will fold removing this smp_mb() in with attribution. Thanx, Paul
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