linux-kernel - [PATCH v4 0/4] LoongArch: Add 128-bit atomic cmpxchg support (v4)

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Message-Id: <20251205-2-v4-0-e5ab932cf219@linux.dev>
Date: Fri, 05 Dec 2025 14:29:03 +0800
From: George Guo <dongtai.guo@...ux.dev>
To: Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org, 
 George Guo <dongtai.guo@...ux.dev>, George Guo <guodongtai@...inos.cn>, 
 Yangyang Lian <lianyangyang@...inos.cn>
Subject: [PATCH v4 0/4] LoongArch: Add 128-bit atomic cmpxchg support (v4)

This patch series adds 128-bit atomic compare-and-exchange support for
LoongArch architecture, which fixes BPF scheduler test failures caused
by missing 128-bit atomics support.

The series consists of four patches:

1. "LoongArch: Add 128-bit atomic cmpxchg support"
   - Implements 128-bit atomic compare-and-exchange using LoongArch's
     LL.D/SC.Q instructions
   - Fixes BPF scheduler test failures (scx_central scx_qmap) where
     kmalloc_nolock_noprof returns NULL due to missing 128-bit atomics,
     leading to -ENOMEM errors during scheduler initialization

2. "LoongArch: Enable 128-bit atomics cmpxchg support"
   - Adds select HAVE_CMPXCHG_DOUBLE and select HAVE_ALIGNED_STRUCT_PAGE
     in Kconfig to enable 128-bit atomic cmpxchg support
3. "LoongArch: Add SCQ support detection"
    - Check CPUCFG2_SCQ bit to determin if the CPU supports
    SCQ instrction.
4. "LoongArch: Use spinlock to emulate 128-bit cmpxchg"
   - For LoongArch CPUs lacking 128-bit atomic instruction(e.g.,
     the SCQ instruction on 3A5000), provide a fallback implementation
     of __cmpxchg128 using a spinlock to emulate the atomic operation.

The issue was identified through BPF scheduler test failures where
scx_central and scx_qmap schedulers would fail to initialize. Testing
was performed using the scx_qmap scheduler from tools/sched_ext/,
confirming that the patches resolve the initialization failures.

Signed-off-by: George Guo <dongtai.guo@...ux.dev>
---
Changes in v4:
- Add SCQ support detection
- Add spinlock to emulate 128-bit cmpxchg
- Link to v3: https://lore.kernel.org/r/20251126-2-v3-0-851b5a516801@linux.dev

Changes in v3:
- dbar 0 -> __WEAK_LLSC_MB
- =ZB" (__ptr[0]) -> "r" (__ptr)
- Link to v2: https://lore.kernel.org/r/20251124-2-v2-0-b38216e25fd9@linux.dev

Changes in v2:
- Use a normal ld.d for the high word instead of ll.d to avoid race
  condition
- Insert a dbar between ll.d and ld.d to prevent reordering
- Simply __cmpxchg128_asm("ll.d", "sc.q", ptr, o, n) to __cmpxchg128_asm(ptr, o, n)
- Fix address operand constraints after testing different approaches:
  * ld.d with "m"
  * ll.d with "ZC",
  * sc.q with "ZB"(alternative constraints caused issues:
   - "r"  caused system hang
   - "ZC" caused compiler error:
     {standard input}: Assembler messages:
     {standard input}:10037: Fatal error: Immediate overflow.
     format: u0:0 )
- Link to v1: https://lore.kernel.org/r/20251120-2-v1-0-705bdc440550@linux.dev

---
George Guo (3):
      LoongArch: Add 128-bit atomic cmpxchg support
      LoongArch: Use spinlock to emulate 128-bit cmpxchg
      LoongArch: Enable 128-bit atomics cmpxchg support

george (1):
      LoongArch: Add SCQ support detection

 arch/loongarch/Kconfig                    |  2 +
 arch/loongarch/include/asm/cmpxchg.h      | 66 +++++++++++++++++++++++++++++++
 arch/loongarch/include/asm/cpu-features.h |  1 +
 arch/loongarch/include/asm/cpu.h          |  2 +
 arch/loongarch/include/asm/loongarch.h    |  1 +
 arch/loongarch/kernel/cpu-probe.c         |  4 ++
 6 files changed, 76 insertions(+)
---
base-commit: 2061f18ad76ecaddf8ed17df81b8611ea88dbddd
change-id: 20251120-2-d03862b2cf6d

Best regards,
-- 
George Guo <dongtai.guo@...ux.dev>