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

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Message-ID: <20251125104316.00001294@linux.dev>
Date: Tue, 25 Nov 2025 10:43:16 +0800
From: George Guo <dongtai.guo@...ux.dev>
To: hev <r@....cc>
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 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
   - 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
> >
> >
>