1.1 --- a/src/share/vm/runtime/objectMonitor.cpp Mon Mar 10 12:58:02 2014 +0100
1.2 +++ b/src/share/vm/runtime/objectMonitor.cpp Thu Mar 13 14:57:01 2014 -0700
1.3 @@ -1,5 +1,5 @@
1.4 /*
1.5 - * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
1.6 + * Copyright (c) 1998, 2014, Oracle and/or its affiliates. All rights reserved.
1.7 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
1.8 *
1.9 * This code is free software; you can redistribute it and/or modify it
1.10 @@ -382,6 +382,12 @@
1.11 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
1.12 if (JvmtiExport::should_post_monitor_contended_enter()) {
1.13 JvmtiExport::post_monitor_contended_enter(jt, this);
1.14 +
1.15 + // The current thread does not yet own the monitor and does not
1.16 + // yet appear on any queues that would get it made the successor.
1.17 + // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
1.18 + // handler cannot accidentally consume an unpark() meant for the
1.19 + // ParkEvent associated with this ObjectMonitor.
1.20 }
1.21
1.22 OSThreadContendState osts(Self->osthread());
1.23 @@ -439,6 +445,12 @@
1.24 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
1.25 if (JvmtiExport::should_post_monitor_contended_entered()) {
1.26 JvmtiExport::post_monitor_contended_entered(jt, this);
1.27 +
1.28 + // The current thread already owns the monitor and is not going to
1.29 + // call park() for the remainder of the monitor enter protocol. So
1.30 + // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
1.31 + // event handler consumed an unpark() issued by the thread that
1.32 + // just exited the monitor.
1.33 }
1.34
1.35 if (event.should_commit()) {
1.36 @@ -1456,6 +1468,14 @@
1.37 // Note: 'false' parameter is passed here because the
1.38 // wait was not timed out due to thread interrupt.
1.39 JvmtiExport::post_monitor_waited(jt, this, false);
1.40 +
1.41 + // In this short circuit of the monitor wait protocol, the
1.42 + // current thread never drops ownership of the monitor and
1.43 + // never gets added to the wait queue so the current thread
1.44 + // cannot be made the successor. This means that the
1.45 + // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1.46 + // consume an unpark() meant for the ParkEvent associated with
1.47 + // this ObjectMonitor.
1.48 }
1.49 if (event.should_commit()) {
1.50 post_monitor_wait_event(&event, 0, millis, false);
1.51 @@ -1499,21 +1519,6 @@
1.52 exit (true, Self) ; // exit the monitor
1.53 guarantee (_owner != Self, "invariant") ;
1.54
1.55 - // As soon as the ObjectMonitor's ownership is dropped in the exit()
1.56 - // call above, another thread can enter() the ObjectMonitor, do the
1.57 - // notify(), and exit() the ObjectMonitor. If the other thread's
1.58 - // exit() call chooses this thread as the successor and the unpark()
1.59 - // call happens to occur while this thread is posting a
1.60 - // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
1.61 - // handler using RawMonitors and consuming the unpark().
1.62 - //
1.63 - // To avoid the problem, we re-post the event. This does no harm
1.64 - // even if the original unpark() was not consumed because we are the
1.65 - // chosen successor for this monitor.
1.66 - if (node._notified != 0 && _succ == Self) {
1.67 - node._event->unpark();
1.68 - }
1.69 -
1.70 // The thread is on the WaitSet list - now park() it.
1.71 // On MP systems it's conceivable that a brief spin before we park
1.72 // could be profitable.
1.73 @@ -1597,6 +1602,33 @@
1.74 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1.75 }
1.76
1.77 + // Without the fix for 8028280, it is possible for the above call:
1.78 + //
1.79 + // Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
1.80 + //
1.81 + // to consume the unpark() that was done when the successor was set.
1.82 + // The solution for this very rare possibility is to redo the unpark()
1.83 + // outside of the JvmtiExport::should_post_monitor_waited() check.
1.84 + //
1.85 + if (node._notified != 0 && _succ == Self) {
1.86 + // In this part of the monitor wait-notify-reenter protocol it
1.87 + // is possible (and normal) for another thread to do a fastpath
1.88 + // monitor enter-exit while this thread is still trying to get
1.89 + // to the reenter portion of the protocol.
1.90 + //
1.91 + // The ObjectMonitor was notified and the current thread is
1.92 + // the successor which also means that an unpark() has already
1.93 + // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1.94 + // consume the unpark() that was done when the successor was
1.95 + // set because the same ParkEvent is shared between Java
1.96 + // monitors and JVM/TI RawMonitors (for now).
1.97 + //
1.98 + // We redo the unpark() to ensure forward progress, i.e., we
1.99 + // don't want all pending threads hanging (parked) with none
1.100 + // entering the unlocked monitor.
1.101 + node._event->unpark();
1.102 + }
1.103 +
1.104 if (event.should_commit()) {
1.105 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
1.106 }
