That's indeed what I thought, thanks!
[email protected], le dim. 08 mars 2026 17:56:12 +0000, a ecrit:
> Hello Samuel,
>
> This is Brent Baccala's AI assistant (Claude) responding to your
> question about the proposed pthread_once fix.
>
> Yes, the code is correct. Here's our analysis:
>
> The original code used atomic_full_barrier() + plain load/store, which
> is correct on x86 but is formally undefined behavior in C11 because
> the plain load on the fast path races with the plain store inside the
> lock. Your fix properly ties the ordering to the accesses themselves.
>
> Three accesses to __run, all correct:
>
> 1. Fast-path read: atomic_load_acquire. If it reads 1, the acquire
> pairs with the atomic_store_release that set it, ensuring all side
> effects of init_routine() are visible to this thread. If it reads 0,
> we fall through to the lock.
>
> 2. Inner read: plain once_control->__run == 0 inside the spinlock.
> This is safe because the only write to __run also happens under the
> spinlock, so the lock serializes them. (The concurrent fast-path
> atomic_load_acquire is a read-read pair, which is not a data race in
> C11.) The spinlock's acquire semantics ensure this read sees the
> latest value written under the lock.
>
> 3. Store: atomic_store_release inside the spinlock. The release
> ordering ensures init_routine() side effects are ordered before __run
> becomes 1. This is the key pairing with the fast-path acquire load.
>
> The original atomic_full_barrier() before the plain read was both too
> strong (full barrier when acquire suffices) and too weak (not tied to
> the access itself, so the compiler could in theory reorder the plain
> load past it, though in practice GCC doesn't). Your version expresses
> the intent precisely.
>
> One minor observation: the inner plain read could optionally be
> atomic_load_relaxed for formal C11 cleanliness, but since it's fully
> protected by the spinlock and only races with other reads, it's not
> necessary. The code is correct as written.
>
> Claude (Brent Baccala's AI assistant)
