Re: [PATCH v3] srcutree: Don't transition atomic SRCU to big in srcu_gp_end()

[Bradley Morgan]

On 11 September 2026 08:21:25 BST, Kunwu Chan <kunwu.chan@xxxxxxxxx> wrote:

Better late than never.

>Atomic SRCU must remain in the small size state. Warn if this
>invariant is violated and avoid transitioning to big in that case.

Fair enough.

Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>


>
>Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
>---
>Changes since v2:
>- Also prevent the transition when the atomic SRCU size-state
>  invariant is violated, per Paul McKenney's suggestion.
>-v2: https://lore.kernel.org/rcu/20260910084629.3695383-1-kunwu.chan@xxxxxxxxx/
>
>Changes since v1:
>- Drop the !is_atomic guard from v1 and add WARN_ON_ONCE() instead,
>  per Paul McKenney's suggestion.
>- v1: https://lore.kernel.org/rcu/20260907075829.2073224-8-kunwu.chan@xxxxxxxxx/
>---
> kernel/rcu/srcutree.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
>diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>index 84c022ec8e09..07db4e459b30 100644
>--- a/kernel/rcu/srcutree.c
>+++ b/kernel/rcu/srcutree.c
>@@ -1073,8 +1073,11 @@ static void srcu_gp_end(struct srcu_struct *ssp, bool is_atomic)
> 		raw_spin_unlock_irq_rcu_node(sup);
> 	}
> 
>-	/* Transition to big if needed. */
>-	if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
>+	/* Transition to big if needed, but never for atomic SRCU. */
>+	if (ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC &&
>+	    ss_state != SRCU_SIZE_SMALL) {
>+		WARN_ON_ONCE(1);
>+	} else if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {

I like this fix.

> 		if (ss_state == SRCU_SIZE_ALLOC)
> 			init_srcu_struct_nodes(ssp, GFP_KERNEL);
> 		else
>

--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@xxxxxxxxx/