Re: [dpdk-stable] [dpdk-dev] [PATCH v1 2/2] rwlock: reimplement with __atomic builtins

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Joyce Kong
> Sent: Thursday, December 13, 2018 3:38 AM
> To: dev@dpdk.org
> Cc: nd@arm.com; Gavin Hu <gavin.hu@arm.com>; thomas@monjalon.net; jerin.jacob@caviumnetworks.com; hemant.agrawal@nxp.com;
> honnappa.nagarahalli@arm.com; joyce.kong@arm.com; stable@dpdk.org
> Subject: [dpdk-dev] [PATCH v1 2/2] rwlock: reimplement with __atomic builtins
> 
> From: Gavin Hu <gavin.hu@arm.com>
> 
> The __sync builtin based implementation generates full memory barriers
> ('dmb ish') on Arm platforms. Using C11 atomic builtins to generate one
> way barriers.
> 
> Here is the assembly code of __sync_compare_and_swap builtin.
> __sync_bool_compare_and_swap(dst, exp, src);
>    0x000000000090f1b0 <+16>:    e0 07 40 f9 ldr x0, [sp, #8]
>    0x000000000090f1b4 <+20>:    e1 0f 40 79 ldrh    w1, [sp, #6]
>    0x000000000090f1b8 <+24>:    e2 0b 40 79 ldrh    w2, [sp, #4]
>    0x000000000090f1bc <+28>:    21 3c 00 12 and w1, w1, #0xffff
>    0x000000000090f1c0 <+32>:    03 7c 5f 48 ldxrh   w3, [x0]
>    0x000000000090f1c4 <+36>:    7f 00 01 6b cmp w3, w1
>    0x000000000090f1c8 <+40>:    61 00 00 54 b.ne    0x90f1d4
> <rte_atomic16_cmpset+52>  // b.any
>    0x000000000090f1cc <+44>:    02 fc 04 48 stlxrh  w4, w2, [x0]
>    0x000000000090f1d0 <+48>:    84 ff ff 35 cbnz    w4, 0x90f1c0
> <rte_atomic16_cmpset+32>
>    0x000000000090f1d4 <+52>:    bf 3b 03 d5 dmb ish
>    0x000000000090f1d8 <+56>:    e0 17 9f 1a cset    w0, eq  // eq = none
> 
> Fixes: af75078faf ("first public release")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
> Reviewed-by: Joyce Kong <joyce.kong@arm.com>
> Tested-by: Joyce Kong <joyce.kong@arm.com>
> ---
>  lib/librte_eal/common/include/generic/rte_rwlock.h | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)

Looks ok to me in general, but I think needs to run extra perf testing
to check for regression/correctness on other platforms (obviously IA is my main concern).
Another thing - my personal preference would be to have it rebased with '_trylock'
functions also updated - for consistency, plus extra functional tests will be available for new method. 

> 
> diff --git a/lib/librte_eal/common/include/generic/rte_rwlock.h b/lib/librte_eal/common/include/generic/rte_rwlock.h
> index 5751a0e..51d3aac 100644
> --- a/lib/librte_eal/common/include/generic/rte_rwlock.h
> +++ b/lib/librte_eal/common/include/generic/rte_rwlock.h
> @@ -64,14 +64,14 @@ rte_rwlock_read_lock(rte_rwlock_t *rwl)
>  	int success = 0;
> 
>  	while (success == 0) {
> -		x = rwl->cnt;
> +		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
>  		/* write lock is held */
>  		if (x < 0) {
>  			rte_pause();
>  			continue;
>  		}
> -		success = rte_atomic32_cmpset((volatile uint32_t *)&rwl->cnt,
> -					      (uint32_t)x, (uint32_t)(x + 1));
> +		success = __atomic_compare_exchange_n(&rwl->cnt, &x, x+1, 1,

As a nit: 'x + 1'


> +					__ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
>  	}
>  }
> 
> @@ -84,7 +84,7 @@ rte_rwlock_read_lock(rte_rwlock_t *rwl)
>  static inline void
>  rte_rwlock_read_unlock(rte_rwlock_t *rwl)
>  {
> -	rte_atomic32_dec((rte_atomic32_t *)(intptr_t)&rwl->cnt);
> +	__atomic_fetch_sub(&rwl->cnt, 1, __ATOMIC_RELEASE);
>  }
> 
>  /**
> @@ -100,14 +100,14 @@ rte_rwlock_write_lock(rte_rwlock_t *rwl)
>  	int success = 0;
> 
>  	while (success == 0) {
> -		x = rwl->cnt;
> +		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
>  		/* a lock is held */
>  		if (x != 0) {
>  			rte_pause();
>  			continue;
>  		}
> -		success = rte_atomic32_cmpset((volatile uint32_t *)&rwl->cnt,
> -					      0, (uint32_t)-1);
> +		success = __atomic_compare_exchange_n(&rwl->cnt, &x, -1, 1,
> +					__ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
>  	}
>  }
> 
> @@ -120,7 +120,7 @@ rte_rwlock_write_lock(rte_rwlock_t *rwl)
>  static inline void
>  rte_rwlock_write_unlock(rte_rwlock_t *rwl)
>  {
> -	rte_atomic32_inc((rte_atomic32_t *)(intptr_t)&rwl->cnt);
> +	__atomic_store_n(&rwl->cnt, 0, __ATOMIC_RELEASE);
>  }
> 
>  /**
> --
> 2.7.4