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

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Mar 03, 2021 at 04:26:31PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> 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@kernel.org>
> ---
>  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.