On Thu, Sep 10, 2026 at 04:46:29PM +0800, Kunwu Chan wrote:
> Atomic SRCU remain in the small size state, so add a
> WARN_ON_ONCE() before the transition check to catch any future
> code path that might violate this invariant.
>
> Signed-off-by: Kunwu Chan <[email protected]>
> ---
> 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/[email protected]/
> ---
> kernel/rcu/srcutree.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 84c022ec8e09..68441c32cebf 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1074,6 +1074,8 @@ static void srcu_gp_end(struct srcu_struct *ssp, bool
> is_atomic)
> }
>
> /* Transition to big if needed. */
> + WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC &&
> + ss_state != SRCU_SIZE_SMALL);
Also much better, but we also need to avoid doing the transition in this
case, because doing that transition breaks atomic SRCU. On way to make
this happen is something like this:
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) {
if (ss_state == SRCU_SIZE_ALLOC)
init_srcu_struct_nodes(ssp, GFP_KERNEL);
Other approaches might take advantage of the fact that WARN_ON_ONCE()
returns the value of its argument, allowing WARN_ON_ONCE() to be used
as a condition in an "if" statement.
Thanx, Paul
> if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
> if (ss_state == SRCU_SIZE_ALLOC)
> init_srcu_struct_nodes(ssp, GFP_KERNEL);
> --
> 2.43.0
>
