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SubjectRe: [RFC PATCH v3 4/5] riscv/cmpxchg: Implement cmpxchg for variables of size 1 and 2
On Fri, Aug 4, 2023 at 4:49 AM Leonardo Bras <leobras@redhat.com> 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@redhat.com>
> ---
> 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

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