8280940: gtest os.release_multi_mappings_vm is racy

Reviewed-by: dcubed, sjohanss

test/hotspot/gtest/runtime/test_os.cpp

@@ -441,39 +441,62 @@ struct NUMASwitcher {
 #endif
 
 #ifndef _AIX // JDK-8257041
-  TEST_VM(os, release_multi_mappings) {
+TEST_VM(os, release_multi_mappings) {
 
   // With NMT enabled, this will trigger JDK-8263464. For now disable the test if NMT=on.
   if (MemTracker::tracking_level() > NMT_off) {
     return;
   }
 
   // Test that we can release an area created with multiple reservation calls
-  const size_t stripe_len = 4 * M;
-  const int num_stripes = 4;
+  // What we do:
+  // A) we reserve 6 small segments (stripes) adjacent to each other. We commit
+  //    them with alternating permissions to prevent the kernel from folding them into
+  //    a single segment.
+  //    -stripe-stripe-stripe-stripe-stripe-stripe-
+  // B) we release the middle four stripes with a single os::release_memory call. This
+  //    tests that os::release_memory indeed works across multiple segments created with
+  //    multiple os::reserve calls.
+  //    -stripe-___________________________-stripe-
+  // C) Into the now vacated address range between the first and the last stripe, we
+  //    re-reserve a new memory range. We expect this to work as a proof that the address
+  //    range was really released by the single release call (B).
+  //
+  // Note that this is inherently racy. Between (B) and (C), some other thread may have
+  //  reserved something into the hole in the meantime. Therefore we keep that range small and
+  //  entrenched between the first and last stripe, which reduces the chance of some concurrent
+  //  thread grabbing that memory.
+
+  const size_t stripe_len = os::vm_allocation_granularity();
+  const int num_stripes = 6;
   const size_t total_range_len = stripe_len * num_stripes;
 
   // reserve address space...
   address p = reserve_multiple(num_stripes, stripe_len);
   ASSERT_NE(p, (address)NULL);
   PRINT_MAPPINGS("A");
 
-  // .. release it...
+  // .. release the middle stripes...
+  address p_middle_stripes = p + stripe_len;
+  const size_t middle_stripe_len = (num_stripes - 2) * stripe_len;
   {
-    // On Windows, use UseNUMAInterleaving=1 which makes
-    //  os::release_memory accept multi-map-ranges.
-    //  Otherwise we would assert (see below for death test).
+    // On Windows, temporarily switch on UseNUMAInterleaving to allow release_memory to release
+    //  multiple mappings in one go (otherwise we assert, which we test too, see death test below).
     WINDOWS_ONLY(NUMASwitcher b(true);)
-    ASSERT_TRUE(os::release_memory((char*)p, total_range_len));
+    ASSERT_TRUE(os::release_memory((char*)p_middle_stripes, middle_stripe_len));
   }
   PRINT_MAPPINGS("B");
 
-  // re-reserve it. This should work unless release failed.
-  address p2 = (address)os::attempt_reserve_memory_at((char*)p, total_range_len);
-  ASSERT_EQ(p2, p);
+  // ...re-reserve the middle stripes. This should work unless release silently failed.
+  address p2 = (address)os::attempt_reserve_memory_at((char*)p_middle_stripes, middle_stripe_len);
+  ASSERT_EQ(p2, p_middle_stripes);
   PRINT_MAPPINGS("C");
 
-  ASSERT_TRUE(os::release_memory((char*)p, total_range_len));
+  // Clean up. Release all mappings.
+  {
+    WINDOWS_ONLY(NUMASwitcher b(true);) // allow release_memory to release multiple regions
+    ASSERT_TRUE(os::release_memory((char*)p, total_range_len));
+  }
 }
 #endif // !AIX