Re: [dpdk-dev] [PATCH 1/1] eal: add 128-bit cmpset (x86-64 only)

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> 
> This operation can be used for non-blocking algorithms, such as a non-
> blocking stack or ring.
> 
> Signed-off-by: Gage Eads <gage.eads@intel.com>
> ---
>  .../common/include/arch/x86/rte_atomic_64.h        | 31 +++++++++++
>  lib/librte_eal/common/include/generic/rte_atomic.h | 65
> ++++++++++++++++++++++
>  2 files changed, 96 insertions(+)
> 
> diff --git a/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> b/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> index fd2ec9c53..b7b90b83e 100644
> --- a/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> +++ b/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> @@ -34,6 +34,7 @@
>  /*
>   * Inspired from FreeBSD src/sys/amd64/include/atomic.h
>   * Copyright (c) 1998 Doug Rabson
> + * Copyright (c) 2019 Intel Corporation
>   * All rights reserved.
>   */
> 
> @@ -46,6 +47,7 @@
> 
>  #include <stdint.h>
>  #include <rte_common.h>
> +#include <rte_compat.h>
>  #include <rte_atomic.h>
> 
>  /*------------------------- 64 bit atomic operations -------------------------*/ @@ -
> 208,4 +210,33 @@ static inline void rte_atomic64_clear(rte_atomic64_t *v)  }
> #endif
> 
> +static inline int __rte_experimental
> +rte_atomic128_cmpset(volatile rte_int128_t *dst,
Does it make sense to call is rte_atomic128_compare_exchange (or ..._cmp_xchg) to indicate it is a compare-exchange operation?

> +		     rte_int128_t *exp, rte_int128_t *src,
> +		     unsigned int weak,
> +		     enum rte_atomic_memmodel_t success,
> +		     enum rte_atomic_memmodel_t failure) {
> +	RTE_SET_USED(weak);
> +	RTE_SET_USED(success);
> +	RTE_SET_USED(failure);
> +	uint8_t res;
> +
> +	asm volatile (
> +		      MPLOCKED
> +		      "cmpxchg16b %[dst];"
> +		      " sete %[res]"
> +		      : [dst] "=m" (dst->val[0]),
> +			"=A" (exp->val[0]),
> +			[res] "=r" (res)
> +		      : "c" (src->val[1]),
> +			"b" (src->val[0]),
> +			"m" (dst->val[0]),
> +			"d" (exp->val[1]),
> +			"a" (exp->val[0])
> +		      : "memory");
> +
> +	return res;
> +}
> +
>  #endif /* _RTE_ATOMIC_X86_64_H_ */
> diff --git a/lib/librte_eal/common/include/generic/rte_atomic.h
> b/lib/librte_eal/common/include/generic/rte_atomic.h
> index b99ba4688..8d612d566 100644
> --- a/lib/librte_eal/common/include/generic/rte_atomic.h
> +++ b/lib/librte_eal/common/include/generic/rte_atomic.h
> @@ -14,6 +14,7 @@
> 
>  #include <stdint.h>
>  #include <rte_common.h>
> +#include <rte_compat.h>
> 
>  #ifdef __DOXYGEN__
> 
> @@ -1082,4 +1083,68 @@ static inline void
> rte_atomic64_clear(rte_atomic64_t *v)  }  #endif
> 
> +/*------------------------ 128 bit atomic operations
> +-------------------------*/
> +
> +/**
> + * 128-bit integer structure.
> + */
> +typedef struct {
> +	uint64_t val[2];
> +} __rte_aligned(16) rte_int128_t;
It looks like '__int128' is available from gcc 4.6. I think we should use '__int128'. We can have it as an internal structure for ease of programming.

> +
> +/**
> + * Memory consistency models used in atomic operations. These control
> +the
> + * behavior of the operation with respect to memory barriers and
> + * thread synchronization.
> + *
> + * These directly match those in the C++11 standard; for details on
> +their
> + * behavior, refer to the standard.
> + */
> +enum rte_atomic_memmodel_t {
> +	RTE_ATOMIC_RELAXED,
> +	RTE_ATOMIC_CONSUME,
> +	RTE_ATOMIC_ACQUIRE,
> +	RTE_ATOMIC_RELEASE,
> +	RTE_ATOMIC_ACQ_REL,
> +	RTE_ATOMIC_SEQ_CST,
> +};
IMO, we can use the GCC provided names. I do not see any advantage to defining our own.

> +
> +/* Only implemented on x86-64 currently. The ifdef prevents compilation
> +from
> + * failing for architectures without a definition of this function.
> + */
Minor comment. We can skip the above comments, the #if below is pretty obvious.

> +#if defined(RTE_ARCH_X86_64)
> +/**
> + * An atomic compare and set function used by the mutex functions.
> + * (atomic) equivalent to:
> + *   if (*dst == exp)
> + *     *dst = src (all 128-bit words)
> + *
> + * @param dst
> + *   The destination into which the value will be written.
> + * @param exp
> + *   Pointer to the expected value. If the operation fails, this memory is
> + *   updated with the actual value.
> + * @param src
> + *   Pointer to the new value.
> + * @param weak
> + *   A value of true allows the comparison to spuriously fail.
> Implementations
> + *   may ignore this argument and only implement the strong variant.
> + * @param success
> + *   If successful, the operation's memory behavior conforms to this (or a
> + *   stronger) model.
> + * @param failure
> + *   If unsuccessful, the operation's memory behavior conforms to this (or a
> + *   stronger) model. This argument cannot be RTE_ATOMIC_RELEASE,
> + *   RTE_ATOMIC_ACQ_REL, or a stronger model than success.
> + * @return
> + *   Non-zero on success; 0 on failure.
> + */
> +static inline int __rte_experimental
> +rte_atomic128_cmpset(volatile rte_int128_t *dst,
> +		     rte_int128_t *exp, rte_int128_t *src,
> +		     unsigned int weak,
> +		     enum rte_atomic_memmodel_t success,
> +		     enum rte_atomic_memmodel_t failure); #endif
> +
>  #endif /* _RTE_ATOMIC_H_ */
> --
> 2.13.6