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/*
* Copyright (c) 1997, 2022, 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_GC_SHARED_GCLOCKER_HPP
#define SHARE_GC_SHARED_GCLOCKER_HPP
#include "gc/shared/gcCause.hpp"
#include "memory/allStatic.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/macros.hpp"
class JavaThread;
// 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
static uint _total_collections; // value for _gc_locker collection
#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);
static bool is_at_safepoint();
public:
// Accessors
static bool is_active() {
assert(GCLocker::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();
// Return true if the designated collection is a GCLocker request
// that should be discarded. Returns true if cause == GCCause::_gc_locker
// and the given total collection value indicates a collection has been
// done since the GCLocker request was made.
static bool should_discard(GCCause::Cause cause, uint total_collections);
// 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.
inline static void lock_critical(JavaThread* thread);
inline static void unlock_critical(JavaThread* thread);
};
#endif // SHARE_GC_SHARED_GCLOCKER_HPP
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