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

[KunWu Chan]

On Sat, Sep 12, 2026 at 12:37 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Fri, Sep 11, 2026 at 03:21:25PM +0800, Kunwu Chan wrote:
> > Atomic SRCU must remain in the small size state. Warn if this
> > invariant is violated and avoid transitioning to big in that case.
> >
> > Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
>
> Very good, queued for further testing and review, thank you!
>
> I folded the new "if" statement onto one line because some years back the
> Linux kernel liberated itself from the 80-column constraint imposed by IBM
> back in 1928.  We can now have up to 100 characters in a line of code.  ;-)

Thanks, Paul. And thanks for sharing that interesting bit of kernel
history! I didn't know about the IBM 80-column constraint from 1928.
I had assumed that 80 columns was still the current convention, so
it's good to know we can use up to 100 now. ;-)

Thanks,
Kunwu

>
>                                                         Thanx, Paul
>
> > ---
> > 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) {
> >               if (ss_state == SRCU_SIZE_ALLOC)
> >                       init_srcu_struct_nodes(ssp, GFP_KERNEL);
> >               else
> > --
> > 2.43.0
> >
> >