author | tschatzl |
Tue, 29 May 2018 09:26:00 +0200 | |
changeset 50605 | 7f63c74f0974 |
parent 49594 | 898ef81cbc0e |
child 53244 | 9807daeb47c4 |
permissions | -rw-r--r-- |
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/* |
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* Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved. |
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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* |
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* This code is free software; you can redistribute it and/or modify it |
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* under the terms of the GNU General Public License version 2 only, as |
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* published by the Free Software Foundation. |
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* |
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* This code is distributed in the hope that it will be useful, but WITHOUT |
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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* version 2 for more details (a copy is included in the LICENSE file that |
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* accompanied this code). |
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* |
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* You should have received a copy of the GNU General Public License version |
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* 2 along with this work; if not, write to the Free Software Foundation, |
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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* |
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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* or visit www.oracle.com if you need additional information or have any |
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* questions. |
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* |
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*/ |
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||
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#ifndef SHARE_VM_GC_SHARED_GCLOCKER_HPP |
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#define SHARE_VM_GC_SHARED_GCLOCKER_HPP |
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#include "memory/allocation.hpp" |
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#include "utilities/globalDefinitions.hpp" |
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#include "utilities/macros.hpp" |
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class JavaThread; |
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// The direct lock/unlock calls do not force a collection if an unlock |
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// decrements the count to zero. Avoid calling these if at all possible. |
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class GCLocker: public AllStatic { |
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private: |
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// The _jni_lock_count keeps track of the number of threads that are |
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// currently in a critical region. It's only kept up to date when |
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// _needs_gc is true. The current value is computed during |
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// safepointing and decremented during the slow path of GCLocker |
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// unlocking. |
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static volatile jint _jni_lock_count; // number of jni active instances. |
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static volatile bool _needs_gc; // heap is filling, we need a GC |
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// note: bool is typedef'd as jint |
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static volatile bool _doing_gc; // unlock_critical() is doing a GC |
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#ifdef ASSERT |
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// This lock count is updated for all operations and is used to |
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// validate the jni_lock_count that is computed during safepoints. |
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static volatile jint _debug_jni_lock_count; |
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#endif |
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// At a safepoint, visit all threads and count the number of active |
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// critical sections. This is used to ensure that all active |
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// critical sections are exited before a new one is started. |
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static void verify_critical_count() NOT_DEBUG_RETURN; |
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static void jni_lock(JavaThread* thread); |
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static void jni_unlock(JavaThread* thread); |
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static bool is_active_internal() { |
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verify_critical_count(); |
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return _jni_lock_count > 0; |
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} |
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static void log_debug_jni(const char* msg); |
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static bool is_at_safepoint(); |
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public: |
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// Accessors |
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static bool is_active() { |
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assert(GCLocker::is_at_safepoint(), "only read at safepoint"); |
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return is_active_internal(); |
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} |
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static bool needs_gc() { return _needs_gc; } |
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// Shorthand |
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static bool is_active_and_needs_gc() { |
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// Use is_active_internal since _needs_gc can change from true to |
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// false outside of a safepoint, triggering the assert in |
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// is_active. |
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return needs_gc() && is_active_internal(); |
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} |
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// In debug mode track the locking state at all times |
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static void increment_debug_jni_lock_count() NOT_DEBUG_RETURN; |
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static void decrement_debug_jni_lock_count() NOT_DEBUG_RETURN; |
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// Set the current lock count |
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static void set_jni_lock_count(int count) { |
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_jni_lock_count = count; |
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verify_critical_count(); |
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} |
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// Sets _needs_gc if is_active() is true. Returns is_active(). |
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static bool check_active_before_gc(); |
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// Stalls the caller (who should not be in a jni critical section) |
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// until needs_gc() clears. Note however that needs_gc() may be |
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// set at a subsequent safepoint and/or cleared under the |
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// JNICritical_lock, so the caller may not safely assert upon |
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// return from this method that "!needs_gc()" since that is |
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// not a stable predicate. |
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static void stall_until_clear(); |
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// The following two methods are used for JNI critical regions. |
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// If we find that we failed to perform a GC because the GCLocker |
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// was active, arrange for one as soon as possible by allowing |
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// all threads in critical regions to complete, but not allowing |
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// other critical regions to be entered. The reasons for that are: |
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// 1) a GC request won't be starved by overlapping JNI critical |
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// region activities, which can cause unnecessary OutOfMemory errors. |
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// 2) even if allocation requests can still be satisfied before GC locker |
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// becomes inactive, for example, in tenured generation possibly with |
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// heap expansion, those allocations can trigger lots of safepointing |
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// attempts (ineffective GC attempts) and require Heap_lock which |
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// slow down allocations tremendously. |
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// |
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// Note that critical regions can be nested in a single thread, so |
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// we must allow threads already in critical regions to continue. |
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// |
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// JNI critical regions are the only participants in this scheme |
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// because they are, by spec, well bounded while in a critical region. |
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// |
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// Each of the following two method is split into a fast path and a |
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// slow path. JNICritical_lock is only grabbed in the slow path. |
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// _needs_gc is initially false and every java thread will go |
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// through the fast path, which simply increments or decrements the |
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// current thread's critical count. When GC happens at a safepoint, |
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// GCLocker::is_active() is checked. Since there is no safepoint in |
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// the fast path of lock_critical() and unlock_critical(), there is |
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// no race condition between the fast path and GC. After _needs_gc |
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// is set at a safepoint, every thread will go through the slow path |
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// after the safepoint. Since after a safepoint, each of the |
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// following two methods is either entered from the method entry and |
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// falls into the slow path, or is resumed from the safepoints in |
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// the method, which only exist in the slow path. So when _needs_gc |
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// is set, the slow path is always taken, till _needs_gc is cleared. |
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inline static void lock_critical(JavaThread* thread); |
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inline static void unlock_critical(JavaThread* thread); |
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static address needs_gc_address() { return (address) &_needs_gc; } |
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}; |
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#endif // SHARE_VM_GC_SHARED_GCLOCKER_HPP |