On Fri, Sep 11, 2026 at 12:58 AM Paul E. McKenney <[email protected]> wrote:
>
> 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);
>
Thanks, Paul. Good point. The original !is_atomic guard in v1
already prevented the transition for atomic SRCU. I was mainly
thinking of the WARN as a diagnostic, since this state should not
occur with the current code. I agree that retaining the protection
against the transition is safer if the invariant is ever violated.
> 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.
>
I prefer the first approach. Using !WARN_ON_ONCE() directly in the
existing condition would make it rather long and less readable,
while the explicit if/else keeps the control flow clear.
Thanks,
Kunwu
> 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
> >
