Re: [PATCH 08/13] srcutree: Forbid srcu_expedite_current() on atomic SRCU

[Paul E. McKenney]

On Mon, Sep 07, 2026 at 03:58:24PM +0800, Kunwu Chan wrote:
> From: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> 
> srcu_expedite_current() has no early guard for atomic SRCU, so calling
> it on an atomic srcu_struct leaks the expediting callback in
> __call_srcu(), leaving ->srcu_ec_state stuck at SRCU_EC_PENDING.
> 
> Add the same WARN_ON_ONCE()-guarded early return used by the other
> forbidden APIs, and list srcu_expedite_current() and srcu_barrier() in
> synchronize_srcu_atomic()'s documentation.
> 
> Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>

Excellent catch, thank you very much!!!  I have queued this to be folded
into the original with attribution, all in the name of bisectability.

							Thanx, Paul

> ---
>  kernel/rcu/srcutree.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 74acd5645d4c..2d2a12f86115 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1972,6 +1972,9 @@ static void srcu_expedite_current_cb(struct rcu_head *rhp)
>   * no current grace period, one might be created.  If the current grace
>   * period is currently sleeping, that sleep will complete before expediting
>   * will take effect.
> + *
> + * This function must not be invoked on srcu_struct structures that are
> + * used with srcu_read_lock_atomic() and synchronize_srcu_atomic().
>   */
>  void srcu_expedite_current(struct srcu_struct *ssp)
>  {
> @@ -1979,6 +1982,9 @@ void srcu_expedite_current(struct srcu_struct *ssp)
>  	bool needcb = false;
>  	struct srcu_data *sdp;
>  
> +	// Atomic SRCU has no callbacks, so there is nothing to expedite.
> +	if (WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC))
> +		return;
>  	migrate_disable();
>  	sdp = this_cpu_ptr(ssp->sda);
>  	raw_spin_lock_irqsave_sdp_contention(sdp, &flags);
> @@ -2104,8 +2110,9 @@ static void srcu_advance_state(struct srcu_struct *ssp, bool is_atomic)
>   *
>   * If synchronize_srcu_atomic() is invoked on a given srcu_struct
>   * structure, then none of call_srcu(), synchronize_srcu(),
> - * synchronize_srcu_expedited(), or start_poll_synchronize_srcu() may be
> - * invoked on that same structure.
> + * synchronize_srcu_expedited(), start_poll_synchronize_srcu(),
> + * srcu_barrier(), or srcu_expedite_current() may be invoked on that
> + * same structure.
>   *
>   * Because synchronize_srcu_atomic() is even more expedited than is
>   * synchronize_srcu_expedited(), there is no expedited counterpart to
> -- 
> 2.43.0
>