linux-kernel - [PATCH v2 0/2] LoongArch: Add 128-bit atomic cmpxchg support (v2)

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Message-Id: <20251124-2-v2-0-b38216e25fd9@linux.dev>
Date: Mon, 24 Nov 2025 17:26:28 +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>
Subject: [PATCH v2 0/2] LoongArch: Add 128-bit atomic cmpxchg support (v2)

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 two 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

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 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 (2):
      LoongArch: Add 128-bit atomic cmpxchg support
      LoongArch: Enable 128-bit atomics cmpxchg support

 arch/loongarch/Kconfig               |  2 ++
 arch/loongarch/include/asm/cmpxchg.h | 47 ++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)
---
base-commit: d5ae5ac32615e4af729f0610fdc11ff4f4798aef
change-id: 20251120-2-d03862b2cf6d

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