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. Thanks, Kunwu > Thanx, Paul
