1 /*
2 * Copyright (c) 2018, 2019, Red Hat, Inc. 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 #ifndef SHARE_GC_SHENANDOAH_SHENANDOAHEVACOOMHANDLER_HPP
26 #define SHARE_GC_SHENANDOAH_SHENANDOAHEVACOOMHANDLER_HPP
27
28 #include "gc/shenandoah/shenandoahPadding.hpp"
29 #include "memory/allocation.hpp"
30 #include "utilities/globalDefinitions.hpp"
31
32 /**
33 * Provides safe handling of out-of-memory situations during evacuation.
34 *
35 * When a Java thread encounters out-of-memory while evacuating an object in a
36 * load-reference-barrier (i.e. it cannot copy the object to to-space), it does not
37 * necessarily follow we can return immediately from the LRB (and store to from-space).
38 *
39 * In very basic case, on such failure we may wait until the the evacuation is over,
40 * and then resolve the forwarded copy, and to the store there. This is possible
41 * because other threads might still have space in their GCLABs, and successfully
42 * evacuate the object.
43 *
44 * But, there is a race due to non-atomic evac_in_progress transition. Consider
45 * thread A is stuck waiting for the evacuation to be over -- it cannot leave with
46 * from-space copy yet. Control thread drops evacuation_in_progress preparing for
47 * next STW phase that has to recover from OOME. Thread B misses that update, and
48 * successfully evacuates the object, does the write to to-copy. But, before
49 * Thread B is able to install the fwdptr, thread A discovers evac_in_progress is
50 * down, exits from here, reads the fwdptr, discovers old from-copy, and stores there.
51 * Thread B then wakes up and installs to-copy. This breaks to-space invariant, and
52 * silently corrupts the heap: we accepted two writes to separate copies of the object.
53 *
54 * The way it is solved here is to maintain a counter of threads inside the
55 * 'evacuation path'. The 'evacuation path' is the part of evacuation that does the actual
56 * allocation, copying and CASing of the copy object, and is protected by this
57 * OOM-during-evac-handler. The handler allows multiple threads to enter and exit
58 * evacuation path, but on OOME it requires all threads that experienced OOME to wait
59 * for current threads to leave, and blocks other threads from entering.
60 *
61 * Detailed state change:
62 *
63 * Upon entry of the evac-path, entering thread will attempt to increase the counter,
64 * using a CAS. Depending on the result of the CAS:
65 * - success: carry on with evac
66 * - failure:
67 * - if offending value is a valid counter, then try again
68 * - if offending value is OOM-during-evac special value: loop until
69 * counter drops to 0, then exit with resolving the ptr
70 *
71 * Upon exit, exiting thread will decrease the counter using atomic dec.
72 *
73 * Upon OOM-during-evac, any thread will attempt to CAS OOM-during-evac
74 * special value into the counter. Depending on result:
75 * - success: busy-loop until counter drops to zero, then exit with resolve
76 * - failure:
77 * - offender is valid counter update: try again
78 * - offender is OOM-during-evac: busy loop until counter drops to
79 * zero, then exit with resolve
80 */
81 class ShenandoahEvacOOMHandler {
82 private:
83 static const jint OOM_MARKER_MASK;
84
85 shenandoah_padding(0);
86 volatile jint _threads_in_evac;
87 shenandoah_padding(1);
88
89 void wait_for_no_evac_threads();
90
91 public:
92 ShenandoahEvacOOMHandler();
93
94 /**
95 * Attempt to enter the protected evacuation path.
96 *
97 * When this returns true, it is safe to continue with normal evacuation.
98 * When this method returns false, evacuation must not be entered, and caller
99 * may safely continue with a simple resolve (if Java thread).
100 */
101 void enter_evacuation();
102
103 /**
104 * Leave evacuation path.
105 */
106 void leave_evacuation();
107
108 /**
109 * Signal out-of-memory during evacuation. It will prevent any other threads
110 * from entering the evacuation path, then wait until all threads have left the
111 * evacuation path, and then return. It is then safe to continue with a simple resolve.
112 */
113 void handle_out_of_memory_during_evacuation();
114
115 void clear();
116 };
117
118 class ShenandoahEvacOOMScope : public StackObj {
119 public:
120 ShenandoahEvacOOMScope();
121 ~ShenandoahEvacOOMScope();
122 };
123
124 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHEVACOOMHANDLER_HPP