Fix race condition in TransitionCollector.

There was a race condition where if multiple threads were calling
TransitionCollector it could cause a crash due to an invalid
collector transition if another thread did the collector transition
before the SuspendAll.

Bug: 13929101
Change-Id: I8c162a83c1f53d0cbdefab62b0a5bcbb151d6c42

1 files changed, 7 insertions, 0 deletions

diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 91f249a..fcf9fe9 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -1408,6 +1408,13 @@ void Heap::TransitionCollector(CollectorType collector_type) {
       MutexLock mu(self, *gc_complete_lock_);
       // Ensure there is only one GC at a time.
       WaitForGcToCompleteLocked(self);
+      // If someone else beat us to it and changed the collector before we could, exit.
+      // This is safe to do before the suspend all since we set the collector_type_running_ before
+      // we exit the loop. If another thread attempts to do the heap transition before we exit,
+      // then it would get blocked on WaitForGcToCompleteLocked.
+      if (collector_type == collector_type_) {
+        return;
+      }
       // GC can be disabled if someone has a used GetPrimitiveArrayCritical but not yet released.
       if (!copying_transition || disable_moving_gc_count_ == 0) {
         // TODO: Not hard code in semi-space collector?