src/hotspot/share/gc/shared/gcLocker.hpp
changeset 47216 71c04702a3d5
parent 35492 c8c0273e6b91
child 49594 898ef81cbc0e
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/gc/shared/gcLocker.hpp	Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,330 @@
+/*
+ * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_VM_GC_SHARED_GCLOCKER_HPP
+#define SHARE_VM_GC_SHARED_GCLOCKER_HPP
+
+#include "gc/shared/collectedHeap.hpp"
+#include "gc/shared/genCollectedHeap.hpp"
+#include "memory/universe.hpp"
+#include "oops/oop.hpp"
+
+// The direct lock/unlock calls do not force a collection if an unlock
+// decrements the count to zero. Avoid calling these if at all possible.
+
+class GCLocker: public AllStatic {
+ private:
+  // The _jni_lock_count keeps track of the number of threads that are
+  // currently in a critical region.  It's only kept up to date when
+  // _needs_gc is true.  The current value is computed during
+  // safepointing and decremented during the slow path of GCLocker
+  // unlocking.
+  static volatile jint _jni_lock_count;  // number of jni active instances.
+  static volatile bool _needs_gc;        // heap is filling, we need a GC
+                                         // note: bool is typedef'd as jint
+  static volatile bool _doing_gc;        // unlock_critical() is doing a GC
+
+#ifdef ASSERT
+  // This lock count is updated for all operations and is used to
+  // validate the jni_lock_count that is computed during safepoints.
+  static volatile jint _debug_jni_lock_count;
+#endif
+
+  // At a safepoint, visit all threads and count the number of active
+  // critical sections.  This is used to ensure that all active
+  // critical sections are exited before a new one is started.
+  static void verify_critical_count() NOT_DEBUG_RETURN;
+
+  static void jni_lock(JavaThread* thread);
+  static void jni_unlock(JavaThread* thread);
+
+  static bool is_active_internal() {
+    verify_critical_count();
+    return _jni_lock_count > 0;
+  }
+
+  static void log_debug_jni(const char* msg);
+ public:
+  // Accessors
+  static bool is_active() {
+    assert(SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
+    return is_active_internal();
+  }
+  static bool needs_gc()       { return _needs_gc;                        }
+
+  // Shorthand
+  static bool is_active_and_needs_gc() {
+    // Use is_active_internal since _needs_gc can change from true to
+    // false outside of a safepoint, triggering the assert in
+    // is_active.
+    return needs_gc() && is_active_internal();
+  }
+
+  // In debug mode track the locking state at all times
+  static void increment_debug_jni_lock_count() NOT_DEBUG_RETURN;
+  static void decrement_debug_jni_lock_count() NOT_DEBUG_RETURN;
+
+  // Set the current lock count
+  static void set_jni_lock_count(int count) {
+    _jni_lock_count = count;
+    verify_critical_count();
+  }
+
+  // Sets _needs_gc if is_active() is true. Returns is_active().
+  static bool check_active_before_gc();
+
+  // Stalls the caller (who should not be in a jni critical section)
+  // until needs_gc() clears. Note however that needs_gc() may be
+  // set at a subsequent safepoint and/or cleared under the
+  // JNICritical_lock, so the caller may not safely assert upon
+  // return from this method that "!needs_gc()" since that is
+  // not a stable predicate.
+  static void stall_until_clear();
+
+  // The following two methods are used for JNI critical regions.
+  // If we find that we failed to perform a GC because the GCLocker
+  // was active, arrange for one as soon as possible by allowing
+  // all threads in critical regions to complete, but not allowing
+  // other critical regions to be entered. The reasons for that are:
+  // 1) a GC request won't be starved by overlapping JNI critical
+  //    region activities, which can cause unnecessary OutOfMemory errors.
+  // 2) even if allocation requests can still be satisfied before GC locker
+  //    becomes inactive, for example, in tenured generation possibly with
+  //    heap expansion, those allocations can trigger lots of safepointing
+  //    attempts (ineffective GC attempts) and require Heap_lock which
+  //    slow down allocations tremendously.
+  //
+  // Note that critical regions can be nested in a single thread, so
+  // we must allow threads already in critical regions to continue.
+  //
+  // JNI critical regions are the only participants in this scheme
+  // because they are, by spec, well bounded while in a critical region.
+  //
+  // Each of the following two method is split into a fast path and a
+  // slow path. JNICritical_lock is only grabbed in the slow path.
+  // _needs_gc is initially false and every java thread will go
+  // through the fast path, which simply increments or decrements the
+  // current thread's critical count.  When GC happens at a safepoint,
+  // GCLocker::is_active() is checked. Since there is no safepoint in
+  // the fast path of lock_critical() and unlock_critical(), there is
+  // no race condition between the fast path and GC. After _needs_gc
+  // is set at a safepoint, every thread will go through the slow path
+  // after the safepoint.  Since after a safepoint, each of the
+  // following two methods is either entered from the method entry and
+  // falls into the slow path, or is resumed from the safepoints in
+  // the method, which only exist in the slow path. So when _needs_gc
+  // is set, the slow path is always taken, till _needs_gc is cleared.
+  static void lock_critical(JavaThread* thread);
+  static void unlock_critical(JavaThread* thread);
+
+  static address needs_gc_address() { return (address) &_needs_gc; }
+};
+
+
+// A NoGCVerifier object can be placed in methods where one assumes that
+// no garbage collection will occur. The destructor will verify this property
+// unless the constructor is called with argument false (not verifygc).
+//
+// The check will only be done in debug mode and if verifygc true.
+
+class NoGCVerifier: public StackObj {
+ friend class PauseNoGCVerifier;
+
+ protected:
+  bool _verifygc;
+  unsigned int _old_invocations;
+
+ public:
+#ifdef ASSERT
+  NoGCVerifier(bool verifygc = true);
+  ~NoGCVerifier();
+#else
+  NoGCVerifier(bool verifygc = true) {}
+  ~NoGCVerifier() {}
+#endif
+};
+
+// A PauseNoGCVerifier is used to temporarily pause the behavior
+// of a NoGCVerifier object. If we are not in debug mode or if the
+// NoGCVerifier object has a _verifygc value of false, then there
+// is nothing to do.
+
+class PauseNoGCVerifier: public StackObj {
+ private:
+  NoGCVerifier * _ngcv;
+
+ public:
+#ifdef ASSERT
+  PauseNoGCVerifier(NoGCVerifier * ngcv);
+  ~PauseNoGCVerifier();
+#else
+  PauseNoGCVerifier(NoGCVerifier * ngcv) {}
+  ~PauseNoGCVerifier() {}
+#endif
+};
+
+
+// A NoSafepointVerifier object will throw an assertion failure if
+// the current thread passes a possible safepoint while this object is
+// instantiated. A safepoint, will either be: an oop allocation, blocking
+// on a Mutex or JavaLock, or executing a VM operation.
+//
+// If StrictSafepointChecks is turned off, it degrades into a NoGCVerifier
+//
+class NoSafepointVerifier : public NoGCVerifier {
+ friend class PauseNoSafepointVerifier;
+
+ private:
+  bool _activated;
+  Thread *_thread;
+ public:
+#ifdef ASSERT
+  NoSafepointVerifier(bool activated = true, bool verifygc = true ) :
+    NoGCVerifier(verifygc),
+    _activated(activated) {
+    _thread = Thread::current();
+    if (_activated) {
+      _thread->_allow_allocation_count++;
+      _thread->_allow_safepoint_count++;
+    }
+  }
+
+  ~NoSafepointVerifier() {
+    if (_activated) {
+      _thread->_allow_allocation_count--;
+      _thread->_allow_safepoint_count--;
+    }
+  }
+#else
+  NoSafepointVerifier(bool activated = true, bool verifygc = true) : NoGCVerifier(verifygc){}
+  ~NoSafepointVerifier() {}
+#endif
+};
+
+// A PauseNoSafepointVerifier is used to temporarily pause the
+// behavior of a NoSafepointVerifier object. If we are not in debug
+// mode then there is nothing to do. If the NoSafepointVerifier
+// object has an _activated value of false, then there is nothing to
+// do for safepoint and allocation checking, but there may still be
+// something to do for the underlying NoGCVerifier object.
+
+class PauseNoSafepointVerifier : public PauseNoGCVerifier {
+ private:
+  NoSafepointVerifier * _nsv;
+
+ public:
+#ifdef ASSERT
+  PauseNoSafepointVerifier(NoSafepointVerifier * nsv)
+    : PauseNoGCVerifier(nsv) {
+
+    _nsv = nsv;
+    if (_nsv->_activated) {
+      _nsv->_thread->_allow_allocation_count--;
+      _nsv->_thread->_allow_safepoint_count--;
+    }
+  }
+
+  ~PauseNoSafepointVerifier() {
+    if (_nsv->_activated) {
+      _nsv->_thread->_allow_allocation_count++;
+      _nsv->_thread->_allow_safepoint_count++;
+    }
+  }
+#else
+  PauseNoSafepointVerifier(NoSafepointVerifier * nsv)
+    : PauseNoGCVerifier(nsv) {}
+  ~PauseNoSafepointVerifier() {}
+#endif
+};
+
+// A SkipGCALot object is used to elide the usual effect of gc-a-lot
+// over a section of execution by a thread. Currently, it's used only to
+// prevent re-entrant calls to GC.
+class SkipGCALot : public StackObj {
+  private:
+   bool _saved;
+   Thread* _t;
+
+  public:
+#ifdef ASSERT
+    SkipGCALot(Thread* t) : _t(t) {
+      _saved = _t->skip_gcalot();
+      _t->set_skip_gcalot(true);
+    }
+
+    ~SkipGCALot() {
+      assert(_t->skip_gcalot(), "Save-restore protocol invariant");
+      _t->set_skip_gcalot(_saved);
+    }
+#else
+    SkipGCALot(Thread* t) { }
+    ~SkipGCALot() { }
+#endif
+};
+
+// JRT_LEAF currently can be called from either _thread_in_Java or
+// _thread_in_native mode. In _thread_in_native, it is ok
+// for another thread to trigger GC. The rest of the JRT_LEAF
+// rules apply.
+class JRTLeafVerifier : public NoSafepointVerifier {
+  static bool should_verify_GC();
+ public:
+#ifdef ASSERT
+  JRTLeafVerifier();
+  ~JRTLeafVerifier();
+#else
+  JRTLeafVerifier() {}
+  ~JRTLeafVerifier() {}
+#endif
+};
+
+// A NoAllocVerifier object can be placed in methods where one assumes that
+// no allocation will occur. The destructor will verify this property
+// unless the constructor is called with argument false (not activated).
+//
+// The check will only be done in debug mode and if activated.
+// Note: this only makes sense at safepoints (otherwise, other threads may
+// allocate concurrently.)
+
+class NoAllocVerifier : public StackObj {
+ private:
+  bool  _activated;
+
+ public:
+#ifdef ASSERT
+  NoAllocVerifier(bool activated = true) {
+    _activated = activated;
+    if (_activated) Thread::current()->_allow_allocation_count++;
+  }
+
+  ~NoAllocVerifier() {
+    if (_activated) Thread::current()->_allow_allocation_count--;
+  }
+#else
+  NoAllocVerifier(bool activated = true) {}
+  ~NoAllocVerifier() {}
+#endif
+};
+
+#endif // SHARE_VM_GC_SHARED_GCLOCKER_HPP