1 /*
2 * Copyright (c) 1997, 2024, 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.
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23 */
24
25 #include "precompiled.hpp"
26 #include "oops/oop.inline.hpp"
27 #include "runtime/basicLock.hpp"
28 #include "runtime/synchronizer.hpp"
29
30 void BasicLock::print_on(outputStream* st, oop owner) const {
31 st->print("monitor");
32 markWord mark_word = displaced_header();
33 if (mark_word.value() != 0) {
34 // Print monitor info if there's an owning oop and it refers to this BasicLock.
35 bool print_monitor_info = (owner != nullptr) && (owner->mark() == markWord::from_pointer((void*)this));
36 mark_word.print_on(st, print_monitor_info);
37 }
38 }
39
40 void BasicLock::move_to(oop obj, BasicLock* dest) {
41 // Check to see if we need to inflate the lock. This is only needed
42 // if an object is locked using "this" lightweight monitor. In that
43 // case, the displaced_header() is unlocked/neutral, because the
44 // displaced_header() contains the header for the originally unlocked
45 // object. However the lock could have already been inflated. But it
46 // does not matter, this inflation will just be a no-op. For other cases,
47 // the displaced header will be either 0x0 or 0x3, which are location
48 // independent, therefore the BasicLock is free to move.
49 //
50 // During OSR we may need to relocate a BasicLock (which contains a
51 // displaced word) from a location in an interpreter frame to a
52 // new location in a compiled frame. "this" refers to the source
53 // BasicLock in the interpreter frame. "dest" refers to the destination
54 // BasicLock in the new compiled frame. We *always* inflate in move_to()
55 // when the object is locked using "this" lightweight monitor.
56 //
57 // The always-Inflate policy works properly, but it depends on the
58 // inflated fast-path operations in fast_lock and fast_unlock to avoid
59 // performance problems. See x86/macroAssembler_x86.cpp: fast_lock()
60 // and fast_unlock() for examples.
61 //
62 // Note that there is a way to safely swing the object's markword from
63 // one stack location to another. This avoids inflation. Obviously,
64 // we need to ensure that both locations refer to the current thread's stack.
65 // There are some subtle concurrency issues, however, and since the benefit is
66 // small (given the support for inflated fast-path locking in the fast_lock, etc)
67 // we'll leave that optimization for another time.
68
69 if (LockingMode == LM_LEGACY) {
70 if (displaced_header().is_neutral()) {
71 // The object is locked and the resulting ObjectMonitor* will also be
72 // locked so it can't be async deflated until ownership is dropped.
73 ObjectSynchronizer::inflate_helper(obj);
74 // WARNING: We cannot put a check here, because the inflation
75 // will not update the displaced header. Once BasicLock is inflated,
76 // no one should ever look at its content.
77 } else {
78 // Typically the displaced header will be 0 (recursive stack lock) or
79 // unused_mark. Naively we'd like to assert that the displaced mark
80 // value is either 0, neutral, or 3. But with the advent of the
81 // store-before-CAS avoidance in fast_lock/compiler_lock_object
82 // we can find any flavor mark in the displaced mark.
83 }
84 dest->set_displaced_header(displaced_header());
85 }
86 #ifdef ASSERT
87 else {
88 dest->set_displaced_header(markWord(badDispHeaderDeopt));
89 }
90 #endif
91 }