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

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Message-ID: <CAEyhmHS_HudrG57i1Y9vVoyEbmMipYnkKnq+RraS_F5LHAVrJg@mail.gmail.com>
Date: Wed, 10 Dec 2025 12:08:24 +0800
From: Hengqi Chen <hengqi.chen@...il.com>
To: George Guo <dongtai.guo@...ux.dev>
Cc: Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, George Guo <guodongtai@...inos.cn>, 
	Yangyang Lian <lianyangyang@...inos.cn>
Subject: Re: [PATCH v4 0/4] LoongArch: Add 128-bit atomic cmpxchg support (v4)

On Fri, Dec 5, 2025 at 2:29 PM George Guo <dongtai.guo@...ux.dev> wrote:
>
> 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.

The scq detection should be the first patch.

> 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>
>
>