1 /*
   2  * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "runtime/basicLock.hpp"
  27 #include "runtime/objectMonitor.hpp"
  28 #include "runtime/synchronizer.hpp"
  29 
  30 void BasicLock::print_on(outputStream* st) const {
  31   st->print("monitor");
  32   markWord mark_word = displaced_header();
  33   if (mark_word.value() != 0)
  34     mark_word.print_on(st);
  35 }
  36 
  37 void BasicLock::move_to(oop obj, BasicLock* dest) {
  38   // Check to see if we need to inflate the lock. This is only needed
  39   // if an object is locked using "this" lightweight monitor. In that
  40   // case, the displaced_header() is unlocked, because the
  41   // displaced_header() contains the header for the originally unlocked
  42   // object. However the object could have already been inflated. But it
  43   // does not matter, the inflation will just a no-op. For other cases,
  44   // the displaced header will be either 0x0 or 0x3, which are location
  45   // independent, therefore the BasicLock is free to move.
  46   //
  47   // During OSR we may need to relocate a BasicLock (which contains a
  48   // displaced word) from a location in an interpreter frame to a
  49   // new location in a compiled frame.  "this" refers to the source
  50   // basiclock in the interpreter frame.  "dest" refers to the destination
  51   // basiclock in the new compiled frame.  We *always* inflate in move_to().
  52   // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
  53   // cause performance problems in code that makes heavy use of a small # of
  54   // uncontended locks.   (We'd inflate during OSR, and then sync performance
  55   // would subsequently plummet because the thread would be forced thru the slow-path).
  56   // This problem has been made largely moot on IA32 by inlining the inflated fast-path
  57   // operations in Fast_Lock and Fast_Unlock in i486.ad.
  58   //
  59   // Note that there is a way to safely swing the object's markword from
  60   // one stack location to another.  This avoids inflation.  Obviously,
  61   // we need to ensure that both locations refer to the current thread's stack.
  62   // There are some subtle concurrency issues, however, and since the benefit is
  63   // is small (given the support for inflated fast-path locking in the fast_lock, etc)
  64   // we'll leave that optimization for another time.
  65 
  66   // Disallow async deflation of the inflated monitor so the
  67   // displaced header stays stable until we've copied it.
  68   ObjectMonitorHandle omh;
  69   if (displaced_header().is_neutral()) {
  70     ObjectSynchronizer::inflate_helper(&omh, obj);
  71     // WARNING: We can not put check here, because the inflation
  72     // will not update the displaced header. Once BasicLock is inflated,
  73     // no one should ever look at its content.
  74   } else {
  75     // Typically the displaced header will be 0 (recursive stack lock) or
  76     // unused_mark.  Naively we'd like to assert that the displaced mark
  77     // value is either 0, neutral, or 3.  But with the advent of the
  78     // store-before-CAS avoidance in fast_lock/compiler_lock_object
  79     // we can find any flavor mark in the displaced mark.
  80   }
  81   dest->set_displaced_header(displaced_header());
  82 }