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?
Thanx, Paul
