Re: [PATCH v3 3/3] rcu: Use _full() API to debug synchronize_rcu()

Boqun Feng
2025-02-26T18:58:32Z

Hi Ulad,

On Tue, Feb 25, 2025 at 12:00:20PM +0100, Uladzislau Rezki (Sony) wrote:
> Switch for using of get_state_synchronize_rcu_full() and
> poll_state_synchronize_rcu_full() pair for debug a normal
> synchronize_rcu() call.
>
> Just using "not" full APIs to identify if a grace period
> is passed or not might lead to a false kernel splat.
>

Could you provide detailed explanation on this? I.e. why is _full() is
needed? I find the current commit message is a bit vague.

Regards,
Boqun

> Link: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/
> Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
> Reported-by: cheung wall <zzqq0103.hey@gmail.com>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
> include/linux/rcupdate_wait.h | 3 +++
> kernel/rcu/tree.c | 8 +++-----
> 2 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
> index f9bed3d3f78d..4c92d4291cce 100644
> --- a/include/linux/rcupdate_wait.h
> +++ b/include/linux/rcupdate_wait.h
> @@ -16,6 +16,9 @@
> struct rcu_synchronize {
> struct rcu_head head;
> struct completion completion;
> +
> + /* This is for debugging. */
> + struct rcu_gp_oldstate oldstate;
> };
> void wakeme_after_rcu(struct rcu_head *head);
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8625f616c65a..48384fa2eaeb 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1632,12 +1632,10 @@ static void rcu_sr_normal_complete(struct llist_node *node)
> {
> struct rcu_synchronize *rs = container_of(
> (struct rcu_head *) node, struct rcu_synchronize, head);
> - unsigned long oldstate = (unsigned long) rs->head.func;
>
> WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
> - !poll_state_synchronize_rcu(oldstate),
> - "A full grace period is not passed yet: %lu",
> - rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
> + !poll_state_synchronize_rcu_full(&rs->oldstate),
> + "A full grace period is not passed yet!\n");
>
> /* Finally. */
> complete(&rs->completion);
> @@ -3247,7 +3245,7 @@ static void synchronize_rcu_normal(void)
> * snapshot before adding a request.
> */
> if (IS_ENABLED(CONFIG_PROVE_RCU))
> - rs.head.func = (void *) get_state_synchronize_rcu();
> + get_state_synchronize_rcu_full(&rs.oldstate);
>
> rcu_sr_normal_add_req(&rs);
>
> --
> 2.39.5
>