1 /*
2 * Copyright (c) 2018, Red Hat, Inc. All rights reserved.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.
7 *
8 * This code is distributed in the hope that it will be useful, but WITHOUT
9 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
10 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
11 * version 2 for more details (a copy is included in the LICENSE file that
12 * accompanied this code).
13 *
14 * You should have received a copy of the GNU General Public License version
15 * 2 along with this work; if not, write to the Free Software Foundation,
16 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
17 *
18 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
19 * or visit www.oracle.com if you need additional information or have any
20 * questions.
21 *
22 */
23
24 #ifndef SHARE_VM_GC_SHENANDOAH_SHENANDOAHEVACOOMHANDLER_HPP
25 #define SHARE_VM_GC_SHENANDOAH_SHENANDOAHEVACOOMHANDLER_HPP
26
27 #include "gc_implementation/shenandoah/shenandoahPadding.hpp"
28
29 /**
30 * Provides safe handling of out-of-memory situations during evacuation.
31 *
32 * When a Java thread encounters out-of-memory while evacuating an object in a
33 * write-barrier (i.e. it cannot copy the object to to-space), it does not necessarily
34 * follow we can return immediately from the WB (and store to from-space).
35 *
36 * In very basic case, on such failure we may wait until the the evacuation is over,
37 * and then resolve the forwarded copy, and to the store there. This is possible
38 * because other threads might still have space in their GCLABs, and successfully
39 * evacuate the object.
40 *
41 * But, there is a race due to non-atomic evac_in_progress transition. Consider
42 * thread A is stuck waiting for the evacuation to be over -- it cannot leave with
43 * from-space copy yet. Control thread drops evacuation_in_progress preparing for
44 * next STW phase that has to recover from OOME. Thread B misses that update, and
45 * successfully evacuates the object, does the write to to-copy. But, before
46 * Thread B is able to install the fwdptr, thread A discovers evac_in_progress is
47 * down, exits from here, reads the fwdptr, discovers old from-copy, and stores there.
48 * Thread B then wakes up and installs to-copy. This breaks to-space invariant, and
49 * silently corrupts the heap: we accepted two writes to separate copies of the object.
50 *
51 * The way it is solved here is to maintain a counter of threads inside the
52 * 'evacuation path'. The 'evacuation path' is the part of evacuation that does the actual
53 * allocation, copying and CASing of the copy object, and is protected by this
54 * OOM-during-evac-handler. The handler allows multiple threads to enter and exit
55 * evacuation path, but on OOME it requires all threads that experienced OOME to wait
56 * for current threads to leave, and blocks other threads from entering.
57 *
58 * Detailed state change:
59 *
60 * Upon entry of the evac-path, entering thread will attempt to increase the counter,
61 * using a CAS. Depending on the result of the CAS:
62 * - success: carry on with evac
63 * - failure:
64 * - if offending value is a valid counter, then try again
65 * - if offending value is OOM-during-evac special value: loop until
66 * counter drops to 0, then exit with read-barrier
67 *
68 * Upon exit, exiting thread will decrease the counter using atomic dec.
69 *
70 * Upon OOM-during-evac, any thread will attempt to CAS OOM-during-evac
71 * special value into the counter. Depending on result:
72 * - success: busy-loop until counter drops to zero, then exit with RB
73 * - failure:
74 * - offender is valid counter update: try again
75 * - offender is OOM-during-evac: busy loop until counter drops to
76 * zero, then exit with RB
77 */
78 class ShenandoahEvacOOMHandler {
79 private:
80 static const jint OOM_MARKER_MASK;
81
82 shenandoah_padding(0);
83 volatile jint _threads_in_evac;
84 shenandoah_padding(1);
85
86 void wait_for_no_evac_threads();
87
88 public:
89 ShenandoahEvacOOMHandler();
90
91 /**
92 * Attempt to enter the protected evacuation path.
93 *
94 * When this returns true, it is safe to continue with normal evacuation.
95 * When this method returns false, evacuation must not be entered, and caller
96 * may safely continue with a read-barrier (if Java thread).
97 */
98 void enter_evacuation();
99
100 /**
101 * Leave evacuation path.
102 */
103 void leave_evacuation();
104
105 /**
106 * Signal out-of-memory during evacuation. It will prevent any other threads
107 * from entering the evacuation path, then wait until all threads have left the
108 * evacuation path, and then return. It is then safe to continue with a read-barrier.
109 */
110 void handle_out_of_memory_during_evacuation();
111
112 void clear();
113 };
114
115 class ShenandoahEvacOOMScope : public StackObj {
116 public:
117 ShenandoahEvacOOMScope();
118 ~ShenandoahEvacOOMScope();
119 };
120
121 #endif // SHARE_VM_GC_SHENANDOAH_SHENANDOAHEVACOOMHANDLER_HPP