[PATCH 00/22] Cleanup internal atomic definitions

[Uros Bizjak]

On Thu, Sep 11, 2025 at 3:52 PM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> This patchset is based on previous work by Wilco [1] [2] to move all
> targets' C11 atomics and remove the unused and redundant atomic
> definitions.
>
> I took a more conservative approach and kept some arch-specific
> definitions on some targets:
>
>   * alpha uses a ‘wmb’ for the atomic_write_barrier, and the instruction
>     does not map to any C11 memory ordering.  Alpha has a peculiar
>     semantic memory, so it's best to keep it.
>
>   * sparc also defines stronger definitions for all barriers.  I am not
>     sure why a strong ordering was used, nor do I have a Sparc machine
>     readily available to check if replacing it with a C11 barrier would
>     be okay, so I will keep it.
>
>   * RISC-V defines optimized atomic_max and atomic_min, which are used in
>     some statistical collection on malloc.  It also defines an
>     atomic_spin_nop, used on pthread spinlocks and adaptive mutexes.
>
>   * x86 defines an atomic_spin_nop.
>
> [1] https://sourceware.org/pipermail/libc-alpha/2022-July/140817.html
> [2] https://sourceware.org/pipermail/libc-alpha/2022-July/140817.html
>
> Adhemerval Zanella (20):
>   aarch64: Remove ununsed atomic macros

Typo: *unused* here and below.

>   arc: Remove ununsed atomic macros
>   csky: Remove ununsed atomic macros
>   mips: Remove ununsed atomic macros
>   or1k: Remove ununsed atomic macros
>   s390: Remove ununsed atomic macros
>   sparc: Remove ununsed atomic macros
>   x86: Remove ununsed atomic macros
>   arm: Consolidade atomic-machine.h and Remove ununsed atomic macros

Typo: *Consolidate* here and below.

>   hppa: Move atomic-machine to generic sysdep
>   m68k: Consolidade atomic-machine.h and Remove ununsed atomic macros
>   loongarch: Consolidate atomic-machine.h and remove ununsed atomic
>     macros
>   powerpc: Consolidate atomic-machine.h
>   riscv: Consolidade atomic-machine.h and remove ununsed atomic macros
>   sh: Move atomic-machine to generic sysdep
>   alpha: Remove USE_ATOMIC_COMPILER_BUILTINS definition
>   microblaze: Remove USE_ATOMIC_COMPILER_BUILTINS definition
>   atomic: Consolidate atomic_full_barrier implementation
>   atomic: Consolidate atomic_read_barrier implementation
>   atomic: Consolidate atomic_write_barrier implementation
>
> Wilco Dijkstra (2):
>   Define __HAVE_64B_ATOMICS from compiler support
>   nptl: Remove ATOMIC_EXCHANGE_USES_CAS usage

Hello Adhemerval!

A patch, following removals of unused target atomic macros could
remove #undefs from <include/atomic.h>:

# undef atomic_compare_and_exchange_val_acq
# undef atomic_compare_and_exchange_val_rel
# undef atomic_compare_and_exchange_bool_acq

Also, I wonder what is the purpose of:

# define atomic_forced_read(x) \
  ({ __typeof (x) __x; __asm ("" : "=r" (__x) : "0" (x)); __x; })

This is *not* a scheduling barrier to prevent the compiler from
reordering memory accesses (let alone the CPU), because it lacks
"memory" clobber and/or volatile qualifier. To guarantee read to a
register, this asm should use "memory" clobber, or it should be
redefined to use atomic_load_acquire().

Uros.