Re: [RFC PATCH v3 4/5] riscv/cmpxchg: Implement cmpxchg for variables of size 1 and 2

[Guo Ren]

On Fri, Aug 4, 2023 at 4:49 AM Leonardo Bras <leobras@xxxxxxxxxx> wrote:
>
> cmpxchg for variables of size 1-byte and 2-bytes is not yet available for
> riscv, even though its present in other architectures such as arm64 and
> x86. This could lead to not being able to implement some locking mechanisms
> or requiring some rework to make it work properly.
>
> Implement 1-byte and 2-bytes cmpxchg in order to achieve parity with other
> architectures.
>
> Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/cmpxchg.h | 35 ++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 5a07646fae65..dfb433ac544f 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -72,6 +72,36 @@
>   * indicated by comparing RETURN with OLD.
>   */
>
> +#define __arch_cmpxchg_mask(sc_sfx, prepend, append, r, p, o, n)       \
> +({                                                                     \
> +       /* Depends on 2-byte variables being 2-byte aligned */          \
> +       ulong __s = ((ulong)(p) & 0x3) * BITS_PER_BYTE;                 \
> +       ulong __mask = GENMASK(((sizeof(*p)) * BITS_PER_BYTE) - 1, 0)   \
> +                       << __s;                                         \
> +       ulong __newx = (ulong)(n) << __s;                               \
> +       ulong __oldx = (ulong)(o) << __s;                               \
> +       ulong __retx;                                                   \
> +       register unsigned int __rc;                                     \
> +                                                                       \
> +       __asm__ __volatile__ (                                          \
> +               prepend                                                 \
> +               "0:     lr.w %0, %2\n"                                  \
> +               "       and  %0, %0, %z5\n"                             \
> +               "       bne  %0, %z3, 1f\n"                             \
bug:
-               "       and  %0, %0, %z5\n"                             \
-               "       bne  %0, %z3, 1f\n"                             \
+               "       and  %1, %0, %z5\n"                             \
+               "       bne  %1, %z3, 1f\n"                             \
Your code breaks the %0.



> +               "       and  %1, %0, %z6\n"                             \
> +               "       or   %1, %1, %z4\n"                             \
> +               "       sc.w" sc_sfx " %1, %1, %2\n"                    \
> +               "       bnez %1, 0b\n"                                  \
> +               append                                                  \
> +               "1:\n"                                                  \
> +               : "=&r" (__retx), "=&r" (__rc), "+A" (*(p))             \
> +               : "rJ" ((long)__oldx), "rJ" (__newx),                   \
> +                 "rJ" (__mask), "rJ" (~__mask)                         \
> +               : "memory");                                            \
> +                                                                       \
> +       r = (__typeof__(*(p)))(__retx >> __s);                          \
> +})
> +
>
>  #define __arch_cmpxchg(lr_sfx, sc_sfx, prepend, append, r, p, co, o, n)        \
>  ({                                                                     \
> @@ -98,6 +128,11 @@
>         __typeof__(*(ptr)) __ret;                                       \
>                                                                         \
>         switch (sizeof(*__ptr)) {                                       \
> +       case 1:                                                         \
> +       case 2:                                                         \
> +               __arch_cmpxchg_mask(sc_sfx, prepend, append,            \
> +                                       __ret, __ptr, __old, __new);    \
> +               break;                                                  \
>         case 4:                                                         \
>                 __arch_cmpxchg(".w", ".w" sc_sfx, prepend, append,      \
>                                 __ret, __ptr, (long), __old, __new);    \
> --
> 2.41.0
>


-- 
Best Regards
 Guo Ren