linux-kernel - Re: [PATCH v3 0/2] LoongArch: Add 128-bit atomic cmpxchg support (v3)

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Message-ID: <CAEyhmHTRTFkMpd7svznM9YKmtNpML4Uz9_wnpizQEA=-T_wgJg@mail.gmail.com>
Date: Thu, 27 Nov 2025 12:15:34 +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>
Subject: Re: [PATCH v3 0/2] LoongArch: Add 128-bit atomic cmpxchg support (v3)

On Wed, Nov 26, 2025 at 7:05 PM Hengqi Chen <hengqi.chen@...il.com> wrote:
>
> On Wed, Nov 26, 2025 at 5:40 PM George Guo <dongtai.guo@...ux.dev> wrote:
> >
> > On Wed, 26 Nov 2025 13:23:57 +0800
> > Hengqi Chen <hengqi.chen@...il.com> wrote:
> >
> > > On Wed, Nov 26, 2025 at 10:06 AM 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 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
> > > >
> > >
> > > This kmalloc_nolock_noprof() was introduced in v6.18-rc1 and has no
> > > caller for now.
>
> OK, it does have a caller in rc-2 [1].
>
>   [1]: https://lore.kernel.org/bpf/20251015000700.28988-1-alexei.starovoitov@gmail.com/
>
> > > Why is this related to the sched_ext failure ?
> > >
> > Hi Hengqi,
> >
> > When running scx_central, function call chain as below:
> > central_init->bpf_timer_init->__bpf_async_init->bpf_map_kmalloc_nolock->kmalloc_nolock
> > ->kmalloc_nolock_noprof
> >
>
> Thanks, will test this series.

I tried with qemu, but it seems the kernel can't even boot.

>
> > The function kmalloc_nolock_noprof returns NULL due to the following
> > condition:
> >
> > if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
> >         /*
> >          * kmalloc_nolock() is not supported on architectures that
> >          * don't implement cmpxchg16b, but debug caches don't use
> >          * per-cpu slab and per-cpu partial slabs. They rely on
> >          * kmem_cache_node->list_lock, so kmalloc_nolock() can
> >          * attempt to allocate from debug caches by
> >          * spin_trylock_irqsave(&n->list_lock, ...)
> >          */
> >         return NULL;
> >
> > The NULL return occurs because kmalloc_nolock is not supported on
> > Loongarch, which don't implement cmpxchg16b. So I am giving the patch.
> >
> > Also I tried with debug caches(CONFIG_SLUB_DEBUG_ON=y), it works,
> > but not a good idea.
> >
> > > > 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 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 (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>
> > > >
> > > >
> >