Function atomic_sporadic_refill_thunk 

pub extern "C" fn atomic_sporadic_refill_thunk(user_data: *mut c_void)

C-callable refill thunk that PlatformTimer::create_periodic invokes from the platform’s timer context. Single atomic store — safe in any thread / ISR context.

Safety

user_data must point at a live AtomicSporadicState; the caller of PlatformTimer::create_periodic owns the lifetime contract.