linux-kernel - Re: [PATCH v2 1/2] LoongArch: Add 128-bit atomic cmpxchg support

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Message-ID: <CAHirt9hAZtp3m5b4htj9R7P+g4xUtPqaRGHtd68NGC+Ke8OnEg@mail.gmail.com>
Date: Tue, 25 Nov 2025 11:32:24 +0800
From: hev <r@....cc>
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 v2 1/2] LoongArch: Add 128-bit atomic cmpxchg support

On Tue, Nov 25, 2025 at 10:43 AM George Guo <dongtai.guo@...ux.dev> wrote:
>
> On Mon, 24 Nov 2025 19:37:40 +0800
> hev <r@....cc> wrote:
>
> > On Mon, Nov 24, 2025 at 5:28 PM George Guo <dongtai.guo@...ux.dev>
> > wrote:
> > >
> > > From: George Guo <guodongtai@...inos.cn>
> > >
> > > Implement 128-bit atomic compare-and-exchange using LoongArch's
> > > LL.D/SC.Q instructions.
> > >
> > > At the same time, fix BPF scheduler test failures (scx_central
> > > scx_qmap) caused by kmalloc_nolock_noprof returning NULL due to
> > > missing 128-bit atomics. The NULL returns led to -ENOMEM errors
> > > during scheduler initialization, causing test cases to fail.
> > >
> > > Verified by testing with the scx_qmap scheduler (located in
> > > tools/sched_ext/). Building with `make` and running
> > > ./tools/sched_ext/build/bin/scx_qmap.
> > >
> > > Signed-off-by: George Guo <guodongtai@...inos.cn>
> > > ---
> > >  arch/loongarch/include/asm/cmpxchg.h | 47
> > > ++++++++++++++++++++++++++++++++++++ 1 file changed, 47
> > > insertions(+)
> > >
> > > diff --git a/arch/loongarch/include/asm/cmpxchg.h
> > > b/arch/loongarch/include/asm/cmpxchg.h index
> > > 979fde61bba8a42cb4f019f13ded2a3119d4aaf4..757f6e82b9880d04f4883dc9a802312111aa4588
> > > 100644 --- a/arch/loongarch/include/asm/cmpxchg.h +++
> > > b/arch/loongarch/include/asm/cmpxchg.h @@ -111,6 +111,44 @@
> > > __arch_xchg(volatile void *ptr, unsigned long x, int size) __ret;
> > >                                                        \ })
> > >
> > > +union __u128_halves {
> > > +       u128 full;
> > > +       struct {
> > > +               u64 low;
> > > +               u64 high;
> > > +       };
> > > +};
> > > +
> > > +#define __cmpxchg128_asm(ptr, old, new)
> > >             \ +({
> > >                   \
> > > +       union __u128_halves __old, __new, __ret;
> > >     \
> > > +       volatile u64 *__ptr = (volatile u64 *)(ptr);
> > >     \
> > > +
> > >     \
> > > +       __old.full = (old);
> > >     \
> > > +       __new.full = (new);
> > >     \
> > > +
> > >     \
> > > +       __asm__ __volatile__(
> > >     \
> > > +       "1:   ll.d    %0, %3            # 128-bit cmpxchg low   \n"
> > >     \
> > > +       "     dbar    0                 # memory barrier        \n"
> > >     \
> > > +       "     ld.d    %1, %4            # 128-bit cmpxchg high  \n"
> > >     \
> > > +       "     bne     %0, %z5, 2f                               \n"
> > >     \
> > > +       "     bne     %1, %z6, 2f                               \n"
> > >     \
> > > +       "     move    $t0, %z7                                  \n"
> > >     \
> > > +       "     move    $t1, %z8                                  \n"
> > >     \
> > > +       "     sc.q    $t0, $t1, %2                              \n"
> > >     \
> > > +       "     beqz    $t0, 1b                                   \n"
> > >     \
> > > +       "2:                                                     \n"
> > >     \
> > > +       __WEAK_LLSC_MB
> > >     \
> > > +       : "=&r" (__ret.low), "=&r" (__ret.high),
> > >     \
> > > +         "=ZB" (__ptr[0])
> > >     \
> >
> > "ZB" isn't a legal constraint for the address operand in sc.q. When
> > assembled, it turns into something like sc.q $r,$r,$r,0, which clearly
> > doesn't match the instruction format, yet gas happily accepts it wheil
> > clang rightfully rejects it. Classic GNU-as leniency biting again. :)
> >
> Hi Hev,
>
> Thanks for your advice, I tried sc.q with r or ZC. the result as
> below: (with gcc 14.2.1 in fedora-42)
>    - sc.q with "r"  caused system hang

input operands:
 : "r" (__ptr), ...

>    - sc.q with "ZC" caused compiler error:
>      {standard input}: Assembler messages:
>      {standard input}:10037: Fatal error: Immediate overflow.
>      format: u0:0 )
> > > +       : "ZC" (__ptr[0]), "m" (__ptr[1]),
> > >     \
> > > +         "Jr" (__old.low), "Jr" (__old.high),
> > >     \
> > > +         "Jr" (__new.low), "Jr" (__new.high)
> > >     \
> > > +       : "t0", "t1", "memory");
> > >     \
> > > +
> > >     \
> > > +       __ret.full;
> > >     \ +})
> > > +
> > >  static inline unsigned int __cmpxchg_small(volatile void *ptr,
> > > unsigned int old, unsigned int new, unsigned int size)
> > >  {
> > > @@ -198,6 +236,15 @@ __cmpxchg(volatile void *ptr, unsigned long
> > > old, unsigned long new, unsigned int __res;
> > >                                  \ })
> > >
> > > +/* cmpxchg128 */
> > > +#define system_has_cmpxchg128()                1
> > > +
> > > +#define arch_cmpxchg128(ptr, o, n)
> > >     \ +({
> > >           \
> > > +       BUILD_BUG_ON(sizeof(*(ptr)) != 16);
> > >     \
> > > +       __cmpxchg128_asm(ptr, o, n);                    \
> > > +})
> > > +
> > >  #ifdef CONFIG_64BIT
> > >  #define arch_cmpxchg64_local(ptr, o, n)
> > >             \ ({
> > >                \
> > >
> > > --
> > > 2.48.1
> > >
> > >
> >
>