Fix spurious wake-ups and worker threads initialization in workerpool

common/workerpool.c

@@ -30,6 +30,7 @@ either expressed or implied, of the Regents of The University of Michigan.
 #define __USE_GNU
 #include "common/pthreads_cross.h"
 #include <assert.h>
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #ifdef _WIN32
@@ -51,6 +52,7 @@ struct workerpool {
 
     pthread_mutex_t mutex;
     pthread_cond_t startcond;   // used to signal the availability of work
+    bool start_predicate;       // predicate that prevents spurious wakeups on startcond
     pthread_cond_t endcond;     // used to signal completion of all work
 
     int end_count; // how many threads are done?
@@ -70,7 +72,7 @@ void *worker_thread(void *p)
         struct task *task;
 
         pthread_mutex_lock(&wp->mutex);
-        while (wp->taskspos == zarray_size(wp->tasks)) {
+        while (wp->taskspos == zarray_size(wp->tasks) || !wp->start_predicate) {
             wp->end_count++;
             pthread_cond_broadcast(&wp->endcond);
             pthread_cond_wait(&wp->startcond, &wp->mutex);
@@ -98,6 +100,7 @@ workerpool_t *workerpool_create(int nthreads)
     workerpool_t *wp = calloc(1, sizeof(workerpool_t));
     wp->nthreads = nthreads;
     wp->tasks = zarray_create(sizeof(struct task));
+    wp->start_predicate = false;
 
     if (nthreads > 1) {
         wp->threads = calloc(wp->nthreads, sizeof(pthread_t));
@@ -114,6 +117,13 @@ workerpool_t *workerpool_create(int nthreads)
                 return NULL;
             }
         }
+
+        // Wait for the worker threads to be ready
+        pthread_mutex_lock(&wp->mutex);
+        while (wp->end_count < wp->nthreads) {
+            pthread_cond_wait(&wp->endcond, &wp->mutex);
+        }
+        pthread_mutex_unlock(&wp->mutex);
     }
 
     return wp;
@@ -130,6 +140,7 @@ void workerpool_destroy(workerpool_t *wp)
             workerpool_add_task(wp, NULL, NULL);
 
         pthread_mutex_lock(&wp->mutex);
+        wp->start_predicate = true;
         pthread_cond_broadcast(&wp->startcond);
         pthread_mutex_unlock(&wp->mutex);
 
@@ -157,7 +168,13 @@ void workerpool_add_task(workerpool_t *wp, void (*f)(void *p), void *p)
     t.f = f;
     t.p = p;
 
-    zarray_add(wp->tasks, &t);
+    if (wp->nthreads > 1) {
+        pthread_mutex_lock(&wp->mutex);
+        zarray_add(wp->tasks, &t);
+        pthread_mutex_unlock(&wp->mutex);
+    } else {
+        zarray_add(wp->tasks, &t);
+    }
 }
 
 void workerpool_run_single(workerpool_t *wp)
@@ -175,19 +192,19 @@ void workerpool_run_single(workerpool_t *wp)
 void workerpool_run(workerpool_t *wp)
 {
     if (wp->nthreads > 1) {
-        wp->end_count = 0;
-
         pthread_mutex_lock(&wp->mutex);
+        wp->end_count = 0;
+        wp->start_predicate = true;
         pthread_cond_broadcast(&wp->startcond);
 
         while (wp->end_count < wp->nthreads) {
 //            printf("caught %d\n", wp->end_count);
             pthread_cond_wait(&wp->endcond, &wp->mutex);
         }
 
-        pthread_mutex_unlock(&wp->mutex);
-
         wp->taskspos = 0;
+        wp->start_predicate = false;
+        pthread_mutex_unlock(&wp->mutex);
 
         zarray_clear(wp->tasks);