Re: [dpdk-dev] [RFC v2] eal: simplify the implementation of rte_ctrl_thread_create

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2021-08-02 07:16, Honnappa Nagarahalli wrote:
> The current described behaviour of rte_ctrl_thread_create is
> rigid which makes the implementation of the function complex.
> The behavior is abstracted to allow for simplified implementation.
> 

Have you considered using a POSIX condition variable instead of atomics 
for synchronization?

> Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> ---
> v2: Used compiler's C++11 atomic built-ins to access the shared variable.
> 
>   lib/eal/common/eal_common_thread.c | 65 +++++++++++++-----------------
>   lib/eal/include/rte_lcore.h        |  8 ++--
>   2 files changed, 32 insertions(+), 41 deletions(-)
> 
> diff --git a/lib/eal/common/eal_common_thread.c b/lib/eal/common/eal_common_thread.c
> index 1a52f42a2b..e3e0bf4bff 100644
> --- a/lib/eal/common/eal_common_thread.c
> +++ b/lib/eal/common/eal_common_thread.c
> @@ -169,35 +169,35 @@ __rte_thread_uninit(void)
>   struct rte_thread_ctrl_params {
>   	void *(*start_routine)(void *);
>   	void *arg;
> -	pthread_barrier_t configured;
> -	unsigned int refcnt;
> +	int ret;
> +	/* Synchronization variable between the control thread
> +	 * and the thread calling rte_ctrl_thread_create function.
> +	 * 0 - Initialized
> +	 * 1 - Control thread is running successfully
> +	 * 2 - Control thread encountered an error. 'ret' has the
> +	 *     error code.
> +	 */
> +	unsigned int sync;
>   };
>   
> -static void ctrl_params_free(struct rte_thread_ctrl_params *params)
> -{
> -	if (__atomic_sub_fetch(&params->refcnt, 1, __ATOMIC_ACQ_REL) == 0) {
> -		(void)pthread_barrier_destroy(&params->configured);
> -		free(params);
> -	}
> -}
> -
>   static void *ctrl_thread_init(void *arg)
>   {
>   	struct internal_config *internal_conf =
>   		eal_get_internal_configuration();
>   	rte_cpuset_t *cpuset = &internal_conf->ctrl_cpuset;
>   	struct rte_thread_ctrl_params *params = arg;
> -	void *(*start_routine)(void *);
> +	void *(*start_routine)(void *) = params->start_routine;
>   	void *routine_arg = params->arg;
>   
>   	__rte_thread_init(rte_lcore_id(), cpuset);
> -
> -	pthread_barrier_wait(&params->configured);
> -	start_routine = params->start_routine;
> -	ctrl_params_free(params);
> -
> -	if (start_routine == NULL)
> +	params->ret = pthread_setaffinity_np(pthread_self(),
> +				sizeof(*cpuset), cpuset);
> +	if (params->ret != 0) {
> +		__atomic_store_n(&params->sync, 2, __ATOMIC_RELEASE);
>   		return NULL;
> +	}
> +
> +	__atomic_store_n(&params->sync, 1, __ATOMIC_RELEASE);
>   
>   	return start_routine(routine_arg);
>   }
> @@ -207,9 +207,6 @@ rte_ctrl_thread_create(pthread_t *thread, const char *name,
>   		const pthread_attr_t *attr,
>   		void *(*start_routine)(void *), void *arg)
>   {
> -	struct internal_config *internal_conf =
> -		eal_get_internal_configuration();
> -	rte_cpuset_t *cpuset = &internal_conf->ctrl_cpuset;
>   	struct rte_thread_ctrl_params *params;
>   	int ret;
>   
> @@ -219,15 +216,12 @@ rte_ctrl_thread_create(pthread_t *thread, const char *name,
>   
>   	params->start_routine = start_routine;
>   	params->arg = arg;
> -	params->refcnt = 2;
> -
> -	ret = pthread_barrier_init(&params->configured, NULL, 2);
> -	if (ret != 0)
> -		goto fail_no_barrier;
> +	params->ret = 0;
> +	params->sync = 0;
>   
>   	ret = pthread_create(thread, attr, ctrl_thread_init, (void *)params);
>   	if (ret != 0)
> -		goto fail_with_barrier;
> +		goto thread_create_failed;
>   
>   	if (name != NULL) {
>   		ret = rte_thread_setname(*thread, name);
> @@ -236,24 +230,21 @@ rte_ctrl_thread_create(pthread_t *thread, const char *name,
>   				"Cannot set name for ctrl thread\n");
>   	}
>   
> -	ret = pthread_setaffinity_np(*thread, sizeof(*cpuset), cpuset);
> -	if (ret != 0)
> -		params->start_routine = NULL;
> +	/* Wait for the control thread to initialize successfully */
> +	while (!__atomic_load_n(&params->sync, __ATOMIC_ACQUIRE))
> +		rte_pause();
> +	ret = params->ret;
>   
> -	pthread_barrier_wait(&params->configured);
> -	ctrl_params_free(params);
> +	free(params);
>   
> -	if (ret != 0)
> -		/* start_routine has been set to NULL above; */
> -		/* ctrl thread will exit immediately */
> +	if (__atomic_load_n(&params->sync, __ATOMIC_ACQUIRE) != 1)
> +		/* ctrl thread is exiting */
>   		pthread_join(*thread, NULL);
>   
>   	return -ret;
>   
> -fail_with_barrier:
> -	(void)pthread_barrier_destroy(&params->configured);
> +thread_create_failed:
>   
> -fail_no_barrier:
>   	free(params);
>   
>   	return -ret;
> diff --git a/lib/eal/include/rte_lcore.h b/lib/eal/include/rte_lcore.h
> index 1550b75da0..f1cc5e38dc 100644
> --- a/lib/eal/include/rte_lcore.h
> +++ b/lib/eal/include/rte_lcore.h
> @@ -420,10 +420,10 @@ rte_thread_unregister(void);
>   /**
>    * Create a control thread.
>    *
> - * Wrapper to pthread_create(), pthread_setname_np() and
> - * pthread_setaffinity_np(). The affinity of the new thread is based
> - * on the CPU affinity retrieved at the time rte_eal_init() was called,
> - * the dataplane and service lcores are then excluded.
> + * Creates a control thread with the given name and attributes. The
> + * affinity of the new thread is based on the CPU affinity retrieved
> + * at the time rte_eal_init() was called, the dataplane and service
> + * lcores are then excluded.
>    *
>    * @param thread
>    *   Filled with the thread id of the new created thread.
>