On Tue, Sep 01, 2026 at 10:00:24AM +0800, KunWu Chan wrote:
> On Tue, Sep 1, 2026 at 9:04 AM Paul E. McKenney <[email protected]> wrote:
> >
> > On Mon, Aug 31, 2026 at 03:49:37PM +0800, Kunwu Chan wrote:
> > > From: Kunwu Chan <[email protected]>
> > >
> > > 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 <[email protected]>
> > > ---
> > > 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.
Sounds good, and I am looking forward to seeing them.
I will admit that I am curious as to why Tiny Atomic SRCU needs different
rcutorture testing than does Tree Atomic SRCU, but I will ask myself
that question again when I see your patches. ;-)
Thanx, Paul
