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.
Thanks.
