park.hpp revision 1879:f95d63e2154a
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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_VM_RUNTIME_PARK_HPP 26#define SHARE_VM_RUNTIME_PARK_HPP 27 28#include "utilities/debug.hpp" 29#include "utilities/globalDefinitions.hpp" 30/* 31 * Per-thread blocking support for JSR166. See the Java-level 32 * Documentation for rationale. Basically, park acts like wait, unpark 33 * like notify. 34 * 35 * 6271289 -- 36 * To avoid errors where an os thread expires but the JavaThread still 37 * exists, Parkers are immortal (type-stable) and are recycled across 38 * new threads. This parallels the ParkEvent implementation. 39 * Because park-unpark allow spurious wakeups it is harmless if an 40 * unpark call unparks a new thread using the old Parker reference. 41 * 42 * In the future we'll want to think about eliminating Parker and using 43 * ParkEvent instead. There's considerable duplication between the two 44 * services. 45 * 46 */ 47 48class Parker : public os::PlatformParker { 49private: 50 volatile int _counter ; 51 Parker * FreeNext ; 52 JavaThread * AssociatedWith ; // Current association 53 54public: 55 Parker() : PlatformParker() { 56 _counter = 0 ; 57 FreeNext = NULL ; 58 AssociatedWith = NULL ; 59 } 60protected: 61 ~Parker() { ShouldNotReachHere(); } 62public: 63 // For simplicity of interface with Java, all forms of park (indefinite, 64 // relative, and absolute) are multiplexed into one call. 65 void park(bool isAbsolute, jlong time); 66 void unpark(); 67 68 // Lifecycle operators 69 static Parker * Allocate (JavaThread * t) ; 70 static void Release (Parker * e) ; 71private: 72 static Parker * volatile FreeList ; 73 static volatile int ListLock ; 74 75}; 76 77///////////////////////////////////////////////////////////// 78// 79// ParkEvents are type-stable and immortal. 80// 81// Lifecycle: Once a ParkEvent is associated with a thread that ParkEvent remains 82// associated with the thread for the thread's entire lifetime - the relationship is 83// stable. A thread will be associated at most one ParkEvent. When the thread 84// expires, the ParkEvent moves to the EventFreeList. New threads attempt to allocate from 85// the EventFreeList before creating a new Event. Type-stability frees us from 86// worrying about stale Event or Thread references in the objectMonitor subsystem. 87// (A reference to ParkEvent is always valid, even though the event may no longer be associated 88// with the desired or expected thread. A key aspect of this design is that the callers of 89// park, unpark, etc must tolerate stale references and spurious wakeups). 90// 91// Only the "associated" thread can block (park) on the ParkEvent, although 92// any other thread can unpark a reachable parkevent. Park() is allowed to 93// return spuriously. In fact park-unpark a really just an optimization to 94// avoid unbounded spinning and surrender the CPU to be a polite system citizen. 95// A degenerate albeit "impolite" park-unpark implementation could simply return. 96// See http://blogs.sun.com/dave for more details. 97// 98// Eventually I'd like to eliminate Events and ObjectWaiters, both of which serve as 99// thread proxies, and simply make the THREAD structure type-stable and persistent. 100// Currently, we unpark events associated with threads, but ideally we'd just 101// unpark threads. 102// 103// The base-class, PlatformEvent, is platform-specific while the ParkEvent is 104// platform-independent. PlatformEvent provides park(), unpark(), etc., and 105// is abstract -- that is, a PlatformEvent should never be instantiated except 106// as part of a ParkEvent. 107// Equivalently we could have defined a platform-independent base-class that 108// exported Allocate(), Release(), etc. The platform-specific class would extend 109// that base-class, adding park(), unpark(), etc. 110// 111// A word of caution: The JVM uses 2 very similar constructs: 112// 1. ParkEvent are used for Java-level "monitor" synchronization. 113// 2. Parkers are used by JSR166-JUC park-unpark. 114// 115// We'll want to eventually merge these redundant facilities and use ParkEvent. 116 117 118class ParkEvent : public os::PlatformEvent { 119 private: 120 ParkEvent * FreeNext ; 121 122 // Current association 123 Thread * AssociatedWith ; 124 intptr_t RawThreadIdentity ; // LWPID etc 125 volatile int Incarnation ; 126 127 // diagnostic : keep track of last thread to wake this thread. 128 // this is useful for construction of dependency graphs. 129 void * LastWaker ; 130 131 public: 132 // MCS-CLH list linkage and Native Mutex/Monitor 133 ParkEvent * volatile ListNext ; 134 ParkEvent * volatile ListPrev ; 135 volatile intptr_t OnList ; 136 volatile int TState ; 137 volatile int Notified ; // for native monitor construct 138 volatile int IsWaiting ; // Enqueued on WaitSet 139 140 141 private: 142 static ParkEvent * volatile FreeList ; 143 static volatile int ListLock ; 144 145 // It's prudent to mark the dtor as "private" 146 // ensuring that it's not visible outside the package. 147 // Unfortunately gcc warns about such usage, so 148 // we revert to the less desirable "protected" visibility. 149 // The other compilers accept private dtors. 150 151 protected: // Ensure dtor is never invoked 152 ~ParkEvent() { guarantee (0, "invariant") ; } 153 154 ParkEvent() : PlatformEvent() { 155 AssociatedWith = NULL ; 156 FreeNext = NULL ; 157 ListNext = NULL ; 158 ListPrev = NULL ; 159 OnList = 0 ; 160 TState = 0 ; 161 Notified = 0 ; 162 IsWaiting = 0 ; 163 } 164 165 // We use placement-new to force ParkEvent instances to be 166 // aligned on 256-byte address boundaries. This ensures that the least 167 // significant byte of a ParkEvent address is always 0. 168 169 void * operator new (size_t sz) ; 170 void operator delete (void * a) ; 171 172 public: 173 static ParkEvent * Allocate (Thread * t) ; 174 static void Release (ParkEvent * e) ; 175} ; 176 177#endif // SHARE_VM_RUNTIME_PARK_HPP 178