[mutex] Create the SharedMutexGuardIf class

It creates a SharedMutexGuard if the boolean parameter in the
constructor is true.

Bug: v8:7790
Change-Id: I77647f648717af05b1239576b690aec6f4e8b268
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2531790
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71162}

src/base/platform/mutex.h

@@ -7,6 +7,7 @@
 
 #include "src/base/base-export.h"
 #include "src/base/lazy-instance.h"
+#include "src/base/optional.h"
 #if V8_OS_WIN
 #include "src/base/win32-headers.h"
 #endif
@@ -340,6 +341,20 @@ class SharedMutexGuard final {
   }
 };
 
+template <MutexSharedType kIsShared,
+          NullBehavior Behavior = NullBehavior::kRequireNotNull>
+class SharedMutexGuardIf final {
+ public:
+  SharedMutexGuardIf(SharedMutex* mutex, bool enable_mutex) {
+    if (enable_mutex) mutex_.emplace(mutex);
+  }
+  SharedMutexGuardIf(const SharedMutexGuardIf&) = delete;
+  SharedMutexGuardIf& operator=(const SharedMutexGuardIf&) = delete;
+
+ private:
+  base::Optional<SharedMutexGuard<kIsShared, Behavior>> mutex_;
+};
+
 }  // namespace base
 }  // namespace v8
 

src/objects/feedback-vector.cc

@@ -502,16 +502,11 @@ void NexusConfig::SetFeedbackPair(FeedbackVector vector,
 
 std::pair<MaybeObject, MaybeObject> NexusConfig::GetFeedbackPair(
     FeedbackVector vector, FeedbackSlot slot) const {
-  if (mode() == BackgroundThread) {
-    isolate()->feedback_vector_access()->LockShared();
-  }
+  base::SharedMutexGuardIf<base::kShared> scope(
+      isolate()->feedback_vector_access(), mode() == BackgroundThread);
   MaybeObject feedback = vector.Get(slot);
   MaybeObject feedback_extra = vector.Get(slot.WithOffset(1));
-  auto return_value = std::make_pair(feedback, feedback_extra);
-  if (mode() == BackgroundThread) {
-    isolate()->feedback_vector_access()->UnlockShared();
-  }
-  return return_value;
+  return std::make_pair(feedback, feedback_extra);
 }
 
 FeedbackNexus::FeedbackNexus(Handle<FeedbackVector> vector, FeedbackSlot slot)

src/objects/transitions.cc

@@ -230,11 +230,9 @@ Map TransitionsAccessor::SearchTransition(Name name, PropertyKind kind,
       return map;
     }
     case kFullTransitionArray: {
-      if (concurrent_access_) isolate_->transition_array_access()->LockShared();
-      Map result = transitions().SearchAndGetTarget(kind, name, attributes);
-      if (concurrent_access_)
-        isolate_->transition_array_access()->UnlockShared();
-      return result;
+      base::SharedMutexGuardIf<base::kShared> scope(
+          isolate_->transition_array_access(), concurrent_access_);
+      return transitions().SearchAndGetTarget(kind, name, attributes);
     }
   }
   UNREACHABLE();