1.1 --- a/src/os/linux/vm/os_linux.cpp Tue Jan 22 05:55:04 2013 -0800
1.2 +++ b/src/os/linux/vm/os_linux.cpp Tue Jan 22 05:56:42 2013 -0800
1.3 @@ -1,5 +1,5 @@
1.4 /*
1.5 - * Copyright (c) 1999, 2012, Oracle and/or its affiliates. All rights reserved.
1.6 + * Copyright (c) 1999, 2013, 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 @@ -5001,11 +5001,12 @@
1.11 }
1.12 -- _nParked ;
1.13
1.14 - // In theory we could move the ST of 0 into _Event past the unlock(),
1.15 - // but then we'd need a MEMBAR after the ST.
1.16 _Event = 0 ;
1.17 status = pthread_mutex_unlock(_mutex);
1.18 assert_status(status == 0, status, "mutex_unlock");
1.19 + // Paranoia to ensure our locked and lock-free paths interact
1.20 + // correctly with each other.
1.21 + OrderAccess::fence();
1.22 }
1.23 guarantee (_Event >= 0, "invariant") ;
1.24 }
1.25 @@ -5068,40 +5069,44 @@
1.26 status = pthread_mutex_unlock(_mutex);
1.27 assert_status(status == 0, status, "mutex_unlock");
1.28 assert (_nParked == 0, "invariant") ;
1.29 + // Paranoia to ensure our locked and lock-free paths interact
1.30 + // correctly with each other.
1.31 + OrderAccess::fence();
1.32 return ret;
1.33 }
1.34
1.35 void os::PlatformEvent::unpark() {
1.36 - int v, AnyWaiters ;
1.37 - for (;;) {
1.38 - v = _Event ;
1.39 - if (v > 0) {
1.40 - // The LD of _Event could have reordered or be satisfied
1.41 - // by a read-aside from this processor's write buffer.
1.42 - // To avoid problems execute a barrier and then
1.43 - // ratify the value.
1.44 - OrderAccess::fence() ;
1.45 - if (_Event == v) return ;
1.46 - continue ;
1.47 - }
1.48 - if (Atomic::cmpxchg (v+1, &_Event, v) == v) break ;
1.49 - }
1.50 - if (v < 0) {
1.51 - // Wait for the thread associated with the event to vacate
1.52 - int status = pthread_mutex_lock(_mutex);
1.53 - assert_status(status == 0, status, "mutex_lock");
1.54 - AnyWaiters = _nParked ;
1.55 - assert (AnyWaiters == 0 || AnyWaiters == 1, "invariant") ;
1.56 - if (AnyWaiters != 0 && WorkAroundNPTLTimedWaitHang) {
1.57 - AnyWaiters = 0 ;
1.58 - pthread_cond_signal (_cond);
1.59 - }
1.60 - status = pthread_mutex_unlock(_mutex);
1.61 - assert_status(status == 0, status, "mutex_unlock");
1.62 - if (AnyWaiters != 0) {
1.63 - status = pthread_cond_signal(_cond);
1.64 - assert_status(status == 0, status, "cond_signal");
1.65 - }
1.66 + // Transitions for _Event:
1.67 + // 0 :=> 1
1.68 + // 1 :=> 1
1.69 + // -1 :=> either 0 or 1; must signal target thread
1.70 + // That is, we can safely transition _Event from -1 to either
1.71 + // 0 or 1. Forcing 1 is slightly more efficient for back-to-back
1.72 + // unpark() calls.
1.73 + // See also: "Semaphores in Plan 9" by Mullender & Cox
1.74 + //
1.75 + // Note: Forcing a transition from "-1" to "1" on an unpark() means
1.76 + // that it will take two back-to-back park() calls for the owning
1.77 + // thread to block. This has the benefit of forcing a spurious return
1.78 + // from the first park() call after an unpark() call which will help
1.79 + // shake out uses of park() and unpark() without condition variables.
1.80 +
1.81 + if (Atomic::xchg(1, &_Event) >= 0) return;
1.82 +
1.83 + // Wait for the thread associated with the event to vacate
1.84 + int status = pthread_mutex_lock(_mutex);
1.85 + assert_status(status == 0, status, "mutex_lock");
1.86 + int AnyWaiters = _nParked;
1.87 + assert(AnyWaiters == 0 || AnyWaiters == 1, "invariant");
1.88 + if (AnyWaiters != 0 && WorkAroundNPTLTimedWaitHang) {
1.89 + AnyWaiters = 0;
1.90 + pthread_cond_signal(_cond);
1.91 + }
1.92 + status = pthread_mutex_unlock(_mutex);
1.93 + assert_status(status == 0, status, "mutex_unlock");
1.94 + if (AnyWaiters != 0) {
1.95 + status = pthread_cond_signal(_cond);
1.96 + assert_status(status == 0, status, "cond_signal");
1.97 }
1.98
1.99 // Note that we signal() _after dropping the lock for "immortal" Events.
1.100 @@ -5187,13 +5192,14 @@
1.101 }
1.102
1.103 void Parker::park(bool isAbsolute, jlong time) {
1.104 + // Ideally we'd do something useful while spinning, such
1.105 + // as calling unpackTime().
1.106 +
1.107 // Optional fast-path check:
1.108 // Return immediately if a permit is available.
1.109 - if (_counter > 0) {
1.110 - _counter = 0 ;
1.111 - OrderAccess::fence();
1.112 - return ;
1.113 - }
1.114 + // We depend on Atomic::xchg() having full barrier semantics
1.115 + // since we are doing a lock-free update to _counter.
1.116 + if (Atomic::xchg(0, &_counter) > 0) return;
1.117
1.118 Thread* thread = Thread::current();
1.119 assert(thread->is_Java_thread(), "Must be JavaThread");
1.120 @@ -5234,6 +5240,8 @@
1.121 _counter = 0;
1.122 status = pthread_mutex_unlock(_mutex);
1.123 assert (status == 0, "invariant") ;
1.124 + // Paranoia to ensure our locked and lock-free paths interact
1.125 + // correctly with each other and Java-level accesses.
1.126 OrderAccess::fence();
1.127 return;
1.128 }
1.129 @@ -5270,12 +5278,14 @@
1.130 _counter = 0 ;
1.131 status = pthread_mutex_unlock(_mutex) ;
1.132 assert_status(status == 0, status, "invariant") ;
1.133 + // Paranoia to ensure our locked and lock-free paths interact
1.134 + // correctly with each other and Java-level accesses.
1.135 + OrderAccess::fence();
1.136 +
1.137 // If externally suspended while waiting, re-suspend
1.138 if (jt->handle_special_suspend_equivalent_condition()) {
1.139 jt->java_suspend_self();
1.140 }
1.141 -
1.142 - OrderAccess::fence();
1.143 }
1.144
1.145 void Parker::unpark() {
