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

[Mattias Rönnblom]

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.nagaraha...@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.