Handle suspend requests in getReferent()

When waiting in getReferent or the like, use a TimedWait, so we
can occasionally check for suspend requests, thus avoiding deadlocks
that can arise from blocking indefinitely in a runnable state.

This is not particularly clean, and may introduce short delays
when we would otherwise deadlock. It's also a bit risky because
we are now releasing the mutator lock in code that previously didn't.

This is a hopefully more correct replacement for aosp/1784003, which
overlooked some of the complications here.

This does not handle a similar problem in the JNI weak reference code.
Each additional use context adds risk here, due to the mutator
lock release.

Bug: 195336624
Bug: 195664026
Test: Build and boot AOSP with much shorter timeouts.
Test: Confirm that the timeout code is invoked.
Change-Id: I0ffb2ffd105bed9dcb8664f92b17cfbcf756a6e0

1 files changed, 2 insertions, 2 deletions

diff --git a/runtime/gc/reference_processor.h b/runtime/gc/reference_processor.h
index 54de5cc572..8ea7bb1297 100644
--- a/runtime/gc/reference_processor.h
+++ b/runtime/gc/reference_processor.h
@@ -58,7 +58,7 @@ class ReferenceProcessor {
   // GetReferent fast path as an optimization.
   void EnableSlowPath() REQUIRES_SHARED(Locks::mutator_lock_);
   void BroadcastForSlowPath(Thread* self);
-  // Decode the referent, may block if references are being processed.
+  // Decode the referent, may block and allow suspension if references are being processed.
   ObjPtr<mirror::Object> GetReferent(Thread* self, ObjPtr<mirror::Reference> reference)
       REQUIRES_SHARED(Locks::mutator_lock_) REQUIRES(!Locks::reference_processor_lock_);
   // Collects the cleared references and returns a task, to be executed after FinishGC, that will
@@ -89,7 +89,7 @@ class ReferenceProcessor {
   // referents.
   void StartPreservingReferences(Thread* self) REQUIRES(!Locks::reference_processor_lock_);
   void StopPreservingReferences(Thread* self) REQUIRES(!Locks::reference_processor_lock_);
-  // Wait until reference processing is done.
+  // Wait until reference processing is done. May temporarily release both required locks.
   void WaitUntilDoneProcessingReferences(Thread* self)
       REQUIRES_SHARED(Locks::mutator_lock_)
       REQUIRES(Locks::reference_processor_lock_);