Safe atomics for profiler map

Author: szegedi

bindings/profilers/wall.cc

@@ -44,13 +44,21 @@ static std::atomic<ProfilerMap*> profilers(new ProfilerMap());
 static void (*old_handler)(int, siginfo_t*, void*) = nullptr;
 
 static void sighandler(int sig, siginfo_t* info, void* context) {
-  auto prof_map = profilers.load();
-  auto prof_it = prof_map->find(Isolate::GetCurrent());
+  auto isolate = Isolate::GetCurrent();
+  WallProfiler* prof = nullptr;
+  auto prof_map = profilers.exchange(nullptr, std::memory_order_acq_rel);
+  if (prof_map) {
+    auto prof_it = prof_map->find(isolate);
+    if (prof_it != prof_map->end()) {
+      prof = prof_it->second;
+    }
+    profilers.store(prof_map, std::memory_order_release);
+  }
   if (old_handler) {
     old_handler(sig, info, context);
   }
-  if (prof_it != prof_map->end()) {
-    prof_it->second->PushContext();
+  if (prof) {
+    prof->PushContext();
   }
 }
 #endif
@@ -383,11 +391,15 @@ WallProfiler::~WallProfiler() {
 
 template <typename F>
 void updateProfilers(F updateFn) {
-  auto currProfilers = profilers.load();
+  auto currProfilers = profilers.load(std::memory_order_acquire);
   for (;;) {
+    while (!currProfilers) {
+      currProfilers = profilers.load(std::memory_order_relaxed);
+    }
     auto newProfilers = new ProfilerMap(*currProfilers);
     updateFn(newProfilers);
-    if (profilers.compare_exchange_weak(currProfilers, newProfilers)) {
+    if (profilers.compare_exchange_weak(
+            currProfilers, newProfilers, std::memory_order_acq_rel)) {
       delete currProfilers;
       break;
     } else {