Re: [PATCH 2/3] rcu: Add warning to ensure rcu_seq_done_exact() is working

[Paul E. McKenney]

On Mon, Mar 24, 2025 at 01:01:54PM -0400, Joel Fernandes wrote:
> The previous patch improved the rcu_seq_done_exact() function by adding
> a meaningful constant for the guardband.
> 
> Ensure that this is working for the future by a quick check during
> rcu_gp_init().
> 
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>

This is a good test for the guardband being way too short.

Are there other tests the should be run, possibly on a separate gp_seq
used only for testing?  Should the test below be under CONFIG_PROVE_RCU?

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 659f83e71048..29ddbcbea25e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1798,6 +1798,7 @@ static noinline_for_stack bool rcu_gp_init(void)
>  	struct rcu_data *rdp;
>  	struct rcu_node *rnp = rcu_get_root();
>  	bool start_new_poll;
> +	unsigned long old_gp_seq;
>  
>  	WRITE_ONCE(rcu_state.gp_activity, jiffies);
>  	raw_spin_lock_irq_rcu_node(rnp);
> @@ -1825,7 +1826,11 @@ static noinline_for_stack bool rcu_gp_init(void)
>  	 */
>  	start_new_poll = rcu_sr_normal_gp_init();
>  	/* Record GP times before starting GP, hence rcu_seq_start(). */
> +	old_gp_seq = rcu_state.gp_seq;
>  	rcu_seq_start(&rcu_state.gp_seq);
> +	/* Ensure that rcu_seq_done_exact() guardband doesn't give false positives. */
> +	WARN_ON_ONCE(rcu_seq_done_exact(&old_gp_seq, rcu_seq_snap(&rcu_state.gp_seq)));
> +
>  	ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
>  	trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
>  	rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
> -- 
> 2.43.0
>