[PATCH 18/22] atomic: Consolidate atomic_full_barrier implementation

[Uros Bizjak]

On Thu, Sep 11, 2025 at 3:53 PM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> All ABIs save for sparcv9 and s390 defines it to __sync_synchronize,
> which can be mapped to __atomic_thread_fence (__ATOMIC_SEQ_CST).
>
> For Sparc, it uses a stricter #StoreStore|#LoadStore|#StoreLoad|#LoadLoad
> instead of the #StoreLoad generated by __sync_synchronize.
>
> For s390x, it defaults to a memory barrier where __sync_synchronize
> emits a 'bcr 15,0' (which the manual describes as pipeline synchronization).
>
> The barrier is used only in one place (pthread_mutex_setprioceiling),
> and using a stricter barrier for s390 is ok performance-wise.
>
> Co-authored-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
> ---
>  include/atomic.h                   |  2 +-
>  sysdeps/aarch64/atomic-machine.h   |  3 ---
>  sysdeps/alpha/atomic-machine.h     |  1 -
>  sysdeps/arc/atomic-machine.h       |  2 --
>  sysdeps/arm/atomic-machine.h       |  2 --
>  sysdeps/generic/malloc-machine.h   |  4 ----
>  sysdeps/hppa/atomic-machine.h      |  2 --
>  sysdeps/loongarch/atomic-machine.h |  2 --
>  sysdeps/mips/atomic-machine.h      | 20 --------------------
>  sysdeps/mips/sys/asm.h             | 16 ----------------
>  sysdeps/or1k/atomic-machine.h      |  2 --
>  sysdeps/powerpc/atomic-machine.h   |  2 --
>  sysdeps/riscv/atomic-machine.h     |  2 --
>  sysdeps/x86/atomic-machine.h       |  1 -
>  14 files changed, 1 insertion(+), 60 deletions(-)
>
> diff --git a/include/atomic.h b/include/atomic.h
> index a9eb4d740b..9bd2753e32 100644
> --- a/include/atomic.h
> +++ b/include/atomic.h
> @@ -103,7 +103,7 @@
>
>
>  #ifndef atomic_full_barrier
> -# define atomic_full_barrier() __asm ("" ::: "memory")
> +# define atomic_full_barrier() __atomic_thread_fence (__ATOMIC_SEQ_CST)
>  #endif

Maybe this change should be more like Wilco's change:

+# define atomic_full_barrier() atomic_thread_fence_seq_cst()

to use macros, defined a few lines below. This is also consistent with
how e.g. atomic_compare_and_exchange_val_acq() uses
atomic_compare_exchange_acquire().

Uros.