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 <joelagn...@nvidia.com>
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
>
