DeterministicSchedule: Deschedule parent threads while they wait to join child threads.

Summary:
Eliminate while loop behavior in DeterministicSchedule::join

Depends on D8789304

Reviewed By: djwatson

Differential Revision: D8840276

fbshipit-source-id: f0b0db60f7bec344d021a49cec408a2604d4a1c8

folly/test/DeterministicSchedule.cpp

@@ -226,6 +226,11 @@ void DeterministicSchedule::afterThreadCreate(sem_t* sem) {
 void DeterministicSchedule::beforeThreadExit() {
   assert(tls_sched == this);
   beforeSharedAccess();
+  auto parent = joins_.find(std::this_thread::get_id());
+  if (parent != joins_.end()) {
+    reschedule(parent->second);
+    joins_.erase(parent);
+  }
   sems_.erase(std::find(sems_.begin(), sems_.end(), tls_sem));
   active_.erase(std::this_thread::get_id());
   if (sems_.size() > 0) {
@@ -242,16 +247,19 @@ void DeterministicSchedule::beforeThreadExit() {
 void DeterministicSchedule::join(std::thread& child) {
   auto sched = tls_sched;
   if (sched) {
-    bool done = false;
-    while (!done) {
-      beforeSharedAccess();
-      done = !sched->active_.count(child.get_id());
-      if (done) {
-        FOLLY_TEST_DSCHED_VLOG("joined " << std::hex << child.get_id());
-      }
+    beforeSharedAccess();
+    assert(sched->joins_.count(child.get_id()) == 0);
+    if (sched->active_.count(child.get_id())) {
+      sem_t* sem = descheduleCurrentThread();
+      sched->joins_.insert({child.get_id(), sem});
       afterSharedAccess();
+      // Wait to be scheduled by exiting child thread
+      beforeSharedAccess();
+      assert(!sched->active_.count(child.get_id()));
     }
+    afterSharedAccess();
   }
+  FOLLY_TEST_DSCHED_VLOG("joined " << std::hex << child.get_id());
   child.join();
 }
 

folly/test/DeterministicSchedule.h

@@ -199,6 +199,7 @@ class DeterministicSchedule : boost::noncopyable {
   std::function<size_t(size_t)> scheduler_;
   std::vector<sem_t*> sems_;
   std::unordered_set<std::thread::id> active_;
+  std::unordered_map<std::thread::id, sem_t*> joins_;
   unsigned nextThreadId_;
   /* step_ keeps count of shared accesses that correspond to user
    * synchronization steps (atomic accesses for now).