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

[Paul E. McKenney]

On Sun, Sep 13, 2026 at 01:47:05PM +0100, Bradley Morgan wrote:
> 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>

Thank you!  I will apply this on my next rebase.

							Thanx, Paul

> >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/