Re: [PATCH v2 -rcu] srcu: Use rcu_seq_done_exact() for polling API

[Boqun Feng]

On Wed, Feb 19, 2025 at 07:43:08AM -0500, Joel Fernandes wrote:
> poll_state_synchronize_srcu() uses rcu_seq_done() unlike
> poll_state_synchronize_rcu() which uses rcu_seq_done_exact().
> 
> The  rcu_seq_done_exact() makes more sense for polling API, as with
> this API, there is a higher chance that there is a significant delay
> between the get_state..() and poll_state..() calls since a cookie
> can be stored and reused at a later time. During such a delay, if
> the gp_seq counter progresses more than ULONG_MAX/2 distance, then
> poll_state..() may return false for a long time unwantedly.
> 
> Fix by using the more accurate rcu_seq_done_exact() API which is
> exactly what straight RCU's polling does.
> 
> It may make sense, as future work, to add debug code here as well, where
> we compare a physical timestamp between get_state..() and poll_state()
> calls and yell if significant time has past but the grace period has
> still not progressed.
> 
> Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>

Queued for further tests and reviews, thank you all.

Regards,
Boqun

> ---
> v1->v2: Resend with Neeraj review tag. Rebased on rcu/dev.
> 
>  kernel/rcu/srcutree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index d2a694944553..591371d62e89 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1589,7 +1589,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
>  bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
>  {
>  	if (cookie != SRCU_GET_STATE_COMPLETED &&
> -	    !rcu_seq_done(&ssp->srcu_sup->srcu_gp_seq, cookie))
> +	    !rcu_seq_done_exact(&ssp->srcu_sup->srcu_gp_seq, cookie))
>  		return false;
>  	// Ensure that the end of the SRCU grace period happens before
>  	// any subsequent code that the caller might execute.
> -- 
> 2.34.1
> 
>