[wasm] Cleanup and document {WasmCodeManager} mutex.

R=clemensh@chromium.org
BUG=v8:8015

Change-Id: I1daca16797a6a359aaab1703767ae24e2d73e8f2
Reviewed-on: https://chromium-review.googlesource.com/1238240Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56123}

src/wasm/wasm-code-manager.cc

@@ -661,8 +661,6 @@ Address NativeModule::AllocateForCode(size_t size) {
         wasm_code_manager_->TryAllocate(size, reinterpret_cast<void*>(hint)));
     VirtualMemory& new_mem = owned_code_space_.back();
     if (!new_mem.IsReserved()) return kNullAddress;
-    base::LockGuard<base::Mutex> lock(
-        &wasm_code_manager_->native_modules_mutex_);
     wasm_code_manager_->AssignRanges(new_mem.address(), new_mem.end(), this);
 
     free_code_space_.Merge({new_mem.address(), new_mem.end()});
@@ -810,7 +808,15 @@ bool WasmCodeManager::Commit(Address start, size_t size) {
 
 void WasmCodeManager::AssignRanges(Address start, Address end,
                                    NativeModule* native_module) {
+  base::LockGuard<base::Mutex> lock(&native_modules_mutex_);
+  lookup_map_.insert(std::make_pair(start, std::make_pair(end, native_module)));
+}
+
+void WasmCodeManager::AssignRangesAndAddModule(Address start, Address end,
+                                               NativeModule* native_module) {
+  base::LockGuard<base::Mutex> lock(&native_modules_mutex_);
   lookup_map_.insert(std::make_pair(start, std::make_pair(end, native_module)));
+  native_modules_.emplace(native_module);
 }
 
 VirtualMemory WasmCodeManager::TryAllocate(size_t size, void* hint) {
@@ -929,9 +935,7 @@ std::unique_ptr<NativeModule> WasmCodeManager::NewNativeModule(
                        std::move(module), env));
   TRACE_HEAP("New NativeModule %p: Mem: %" PRIuPTR ",+%zu\n", this, start,
              size);
-  base::LockGuard<base::Mutex> lock(&native_modules_mutex_);
-  AssignRanges(start, end, ret.get());
-  native_modules_.emplace(ret.get());
+  AssignRangesAndAddModule(start, end, ret.get());
   return ret;
 }
 

src/wasm/wasm-code-manager.h

@@ -469,13 +469,21 @@ class V8_EXPORT_PRIVATE WasmCodeManager final {
   void FreeNativeModule(NativeModule*);
   void Free(VirtualMemory* mem);
   void AssignRanges(Address start, Address end, NativeModule*);
+  void AssignRangesAndAddModule(Address start, Address end, NativeModule*);
   bool ShouldForceCriticalMemoryPressureNotification();
 
   WasmMemoryTracker* const memory_tracker_;
+  std::atomic<size_t> remaining_uncommitted_code_space_;
   mutable base::Mutex native_modules_mutex_;
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Protected by {native_modules_mutex_}:
+
   std::map<Address, std::pair<Address, NativeModule*>> lookup_map_;
   std::unordered_set<NativeModule*> native_modules_;
-  std::atomic<size_t> remaining_uncommitted_code_space_;
+
+  // End of fields protected by {native_modules_mutex_}.
+  //////////////////////////////////////////////////////////////////////////////
 
   DISALLOW_COPY_AND_ASSIGN(WasmCodeManager);
 };