seqlock: Enforce C11 Atomics

[25077667]

Pull Request Summary

This pull request aims to enhance the existing seqlock implementation by making it more portable, improving code readability, and adhering to coding standards. The changes involve modifications to both the Makefile and the seqlock code itself.

Changes Made

Makefile Enhancements

• Added compilation flags to improve code quality and compliance:
  • -Wextra: This flag enables additional warnings beyond -Wall, helping catch potential issues in the code.
  • -pedantic: Enforced strict adherence to the C standard, promoting standardized and portable code.

Code Improvements

• Transitioned to using <stdatomic.h> for atomic operations, which enhances synchronization and makes the code more standardized.
• Employed atomic_thread_fence for memory barriers, enhancing portability across different architectures.
• Replaced architecture-dependent inline assembly instructions with standardized atomic operations. This improves cross-architecture compatibility and maintainability.
• Revised the ATOMIC_COPY macro to utilize atomic load and store operations for atomic data copying.

Data Types and Typedefs

• Redefined the seqlock_t type using _Atomic(uint32_t) to ensure atomic access to the variable, further improving synchronization.

Motivation

These enhancements were motivated by the goal of creating a more reliable and maintainable seqlock implementation. The changes aim to address portability concerns across various architectures while improving the overall readability and standardization of the codebase.

Request for Review

Kindly review the changes made in this pull request. Your feedback is highly valuable, and any suggestions for further improvements are welcome.

Thank you for considering this pull request.

Best regards,
scc
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[jserv]

You have to confirm the generated code is identical to the intended instruction sequence.

[25077667]

It would be generating

dmb     ish

with Apple clang version 14.0.3 (clang-1403.0.22.14.1).

I would figure what's the output in gcc x86

[25077667]

In gcc 13.2.1 with x86-64, both

1. using __builtin_ia32_pause()
2. using atomic_thread_fence(memory_order_seq_cst)

are generating

data16 cs nopw 0x0(%rax,%rax,1)
nop
xchg   %ax,%ax

[25077667]

In LLVM clang version 15.0.7 with x86-64, adding an extra pause when __builtin_ia32_pause is used.

    125 0000000000001190 <seqlock_acquire_rd>:
    126     1190:       50                      push   %rax
    127     1191:       0f ae f0                mfence
    128     1194:       8b 07                   mov    (%rdi),%eax
    129     1196:       a8 01                   test   $0x1,%al
    130     1198:       75 08                   jne    11a2 <seqlock_acquire_rd+0x12>
    131     119a:       a8 01                   test   $0x1,%al
    132     119c:       75 0f                   jne    11ad <seqlock_acquire_rd+0x1d>
    133     119e:       59                      pop    %rcx
    134     119f:       c3                      ret
    135     11a0:       f3 90                   pause
    136     11a2:       0f ae f0                mfence
    137     11a5:       8b 07                   mov    (%rdi),%eax
    138     11a7:       a8 01                   test   $0x1,%al
    139     11a9:       75 f5                   jne    11a0 <seqlock_acquire_rd+0x10>
    140     11ab:       eb ed                   jmp    119a <seqlock_acquire_rd+0xa>
    141     11ad:       48 8d 3d 6f 0e 00 00    lea    0xe6f(%rip),%rdi        # 2023 <_IO_stdin_used+0x23>
    142     11b4:       48 8d 35 82 0e 00 00    lea    0xe82(%rip),%rsi        # 203d <_IO_stdin_used+0x3d>
    143     11bb:       48 8d 0d 85 0e 00 00    lea    0xe85(%rip),%rcx        # 2047 <_IO_stdin_used+0x47>
    144     11c2:       ba 40 00 00 00          mov    $0x40,%edx
    145     11c7:       e8 84 fe ff ff          call   1050 <__assert_fail@plt>
    146     11cc:       0f 1f 40 00             nopl   0x0(%rax)

[jserv]

Remove seqlock/tests, which was generated.

[25077667]

Remove seqlock/tests, which was generated.

I'm sorry for that commit. I fixed it.

[jserv]

Change to #if !defined(__aarch64) for generic C11 Atomics path.