7143858: G1: Back to back young GCs with the second GC having a minimally sized eden

Summary: Before the last thread to leave a JNI critical region was able to schedule a GCLocker Initiated GC, another thread was attempting an allocation and saw that the GCLocker region was no longer active and successfully scheduled a GC. Stall allocating threads until the GCLocker Initiated GC is performed and then retry the allocation.
Reviewed-by: brutisso, huntch

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -952,9 +952,18 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size,
         }
         should_try_gc = false;
       } else {
-        // Read the GC count while still holding the Heap_lock.
-        gc_count_before = total_collections();
-        should_try_gc = true;
+        // The GCLocker may not be active but the GCLocker initiated
+        // GC may not yet have been performed (GCLocker::needs_gc()
+        // returns true). In this case we do not try this GC and
+        // wait until the GCLocker initiated GC is performed, and
+        // then retry the allocation.
+        if (GC_locker::needs_gc()) {
+          should_try_gc = false;
+        } else {
+          // Read the GC count while still holding the Heap_lock.
+          gc_count_before = total_collections();
+          should_try_gc = true;
+        }
       }
     }
 
@@ -975,6 +984,9 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size,
         return NULL;
       }
     } else {
+      // The GCLocker is either active or the GCLocker initiated
+      // GC has not yet been performed. Stall until it is and
+      // then retry the allocation.
       GC_locker::stall_until_clear();
     }
 
@@ -1054,9 +1066,18 @@ HeapWord* G1CollectedHeap::attempt_allocation_humongous(size_t word_size,
       if (GC_locker::is_active_and_needs_gc()) {
         should_try_gc = false;
       } else {
-        // Read the GC count while still holding the Heap_lock.
-        gc_count_before = total_collections();
-        should_try_gc = true;
+         // The GCLocker may not be active but the GCLocker initiated
+        // GC may not yet have been performed (GCLocker::needs_gc()
+        // returns true). In this case we do not try this GC and
+        // wait until the GCLocker initiated GC is performed, and
+        // then retry the allocation.
+        if (GC_locker::needs_gc()) {
+          should_try_gc = false;
+        } else {
+          // Read the GC count while still holding the Heap_lock.
+          gc_count_before = total_collections();
+          should_try_gc = true;
+        }
       }
     }
 
@@ -1081,6 +1102,9 @@ HeapWord* G1CollectedHeap::attempt_allocation_humongous(size_t word_size,
         return NULL;
       }
     } else {
+      // The GCLocker is either active or the GCLocker initiated
+      // GC has not yet been performed. Stall until it is and
+      // then retry the allocation.
       GC_locker::stall_until_clear();
     }