Re: [PATCH] srcutiny: Add atomic SRCU operation checks and state

[KunWu Chan]

On Tue, Sep 1, 2026 at 9:04 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Mon, Aug 31, 2026 at 03:49:37PM +0800, Kunwu Chan wrote:
> > From: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> >
> > Add atomic SRCU operation checks and the associated state to Tiny
> > SRCU.
> >
> > An atomic SRCU domain does not use the normal SRCU callback and
> > grace-period machinery.  In particular, a callback queued with
> > call_srcu() would never be processed.  Use WARN_ON_ONCE() to reject
> > call_srcu() and srcu_barrier() on atomic SRCU domains.
> >
> > For synchronize_srcu(), redirect atomic SRCU domains to
> > synchronize_srcu_atomic().
> >
> > Add srcu_reader_flavor to the Tiny SRCU state for these checks.
> > Tiny SRCU does not currently set the flavor for atomic SRCU domains,
> > but keeping the flavor in the common state allows the operation
> > checks to enforce the restriction once atomic flavor tracking is
> > enabled.
> >
> > Also initialize srcu_atomic_gp_flag, which was previously left
> > uninitialized.
> >
> > Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> > ---
> >  include/linux/srcutiny.h |  1 +
> >  kernel/rcu/srcutiny.c    | 12 ++++++++++++
> >  2 files changed, 13 insertions(+)
> >
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index 47a368f945e3..2b293336525a 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -20,6 +20,7 @@ struct srcu_struct {
> >       u8 srcu_gp_running;             /* GP workqueue running? */
> >       u8 srcu_gp_waiting;             /* GP waiting for readers? */
> >       u8 srcu_atomic_gp_flag;         /* Serialize atomic GP work.*/
> > +     u8 srcu_reader_flavor;          /* Values: SRCU_READ_FLAVOR_.*  */
> >       unsigned long srcu_idx;         /* Current reader array element in bit 0x2. */
> >       unsigned long srcu_idx_max;     /* Furthest future srcu_idx request. */
> >       struct swait_queue_head srcu_wq;
> > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > index 26ea4bfbeaf2..22f7716cbb0e 100644
> > --- a/kernel/rcu/srcutiny.c
> > +++ b/kernel/rcu/srcutiny.c
> > @@ -41,6 +41,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> >       ssp->srcu_cb_tail = &ssp->srcu_cb_head;
> >       ssp->srcu_gp_running = false;
> >       ssp->srcu_gp_waiting = false;
> > +     ssp->srcu_atomic_gp_flag = 0;
>
> Good catch!  I will be folding this into the base commit with attribution
> on my next rebase:
>
> 9a2e9996ccec ("srcutiny: Add an atomic Tiny SRCU")
>
> >       ssp->srcu_idx = 0;
> >       ssp->srcu_idx_max = 0;
> >       INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> > @@ -289,6 +290,9 @@ EXPORT_SYMBOL_GPL(srcu_defer_drain);
> >  void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >              rcu_callback_t func)
> >  {
> > +     if (WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC))
> > +             return;
> > +
> >       if (should_rcu_defer()) {
> >               /* A re-entrant call_srcu() during the drain would livelock it. */
> >               if (READ_ONCE(srcu_defer_draining) && !in_nmi()) {
> > @@ -319,6 +323,11 @@ void synchronize_srcu(struct srcu_struct *ssp)
> >  {
> >       struct rcu_synchronize rs;
> >
> > +     if (WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC)) {
> > +             synchronize_srcu_atomic(ssp);
> > +             return;
> > +     }
> > +
> >       srcu_lock_sync(&ssp->dep_map);
> >
> >       RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
> > @@ -415,6 +424,9 @@ EXPORT_SYMBOL_GPL(synchronize_srcu_atomic);
> >  /* Register any deferred callbacks, then wait for all in-flight ones. */
> >  void srcu_barrier(struct srcu_struct *ssp)
> >  {
> > +     if (WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC))
> > +             return;
> > +
> >       __srcu_defer_drain(ssp);
> >       synchronize_srcu(ssp);
> >  }
>
> The rest is good as far as it goes, but don't we need to set the value
> of ssp->srcu_reader_flavor somewhere for atomic srcu_struct structures?
>

Hi Paul,

Yes, agreed. I have the flavor tracking changes implemented locally
and am testing them now.
I’ll send the updated patch shortly.

I also have draft patches for the documentation and the fast path, as
well as rcutorture testing for
tiny atomic srcu , which I’ll send separately.

Thanks,
Kunwu

>                                                         Thanx, Paul