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platform_system_core/libutils/RefBase.cpp

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/*
* Copyright (C) 2005 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#define LOG_TAG "RefBase"
// #define LOG_NDEBUG 0
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <typeinfo>
#include <unistd.h>
#include <utils/RefBase.h>
#include <utils/CallStack.h>
#include <utils/Log.h>
#include <utils/threads.h>
#ifndef __unused
#define __unused __attribute__((__unused__))
#endif
// compile with refcounting debugging enabled
#define DEBUG_REFS 0
// whether ref-tracking is enabled by default, if not, trackMe(true, false)
// needs to be called explicitly
#define DEBUG_REFS_ENABLED_BY_DEFAULT 0
// whether callstack are collected (significantly slows things down)
#define DEBUG_REFS_CALLSTACK_ENABLED 1
// folder where stack traces are saved when DEBUG_REFS is enabled
// this folder needs to exist and be writable
#define DEBUG_REFS_CALLSTACK_PATH "/data/debug"
// log all reference counting operations
#define PRINT_REFS 0
// ---------------------------------------------------------------------------
namespace android {
// Usage, invariants, etc:
// It is normally OK just to keep weak pointers to an object. The object will
// be deallocated by decWeak when the last weak reference disappears.
// Once a a strong reference has been created, the object will disappear once
// the last strong reference does (decStrong).
// AttemptIncStrong will succeed if the object has a strong reference, or if it
// has a weak reference and has never had a strong reference.
// AttemptIncWeak really does succeed only if there is already a WEAK
// reference, and thus may fail when attemptIncStrong would succeed.
// OBJECT_LIFETIME_WEAK changes this behavior to retain the object
// unconditionally until the last reference of either kind disappears. The
// client ensures that the extendObjectLifetime call happens before the dec
// call that would otherwise have deallocated the object, or before an
// attemptIncStrong call that might rely on it. We do not worry about
// concurrent changes to the object lifetime.
// mStrong is the strong reference count. mWeak is the weak reference count.
// Between calls, and ignoring memory ordering effects, mWeak includes strong
// references, and is thus >= mStrong.
//
// A weakref_impl is allocated as the value of mRefs in a RefBase object on
// construction.
// In the OBJECT_LIFETIME_STRONG case, it is deallocated in the RefBase
// destructor iff the strong reference count was never incremented. The
// destructor can be invoked either from decStrong, or from decWeak if there
// was never a strong reference. If the reference count had been incremented,
// it is deallocated directly in decWeak, and hence still lives as long as
// the last weak reference.
// In the OBJECT_LIFETIME_WEAK case, it is always deallocated from the RefBase
// destructor, which is always invoked by decWeak. DecStrong explicitly avoids
// the deletion in this case.
//
// Memory ordering:
// The client must ensure that every inc() call, together with all other
// accesses to the object, happens before the corresponding dec() call.
//
// We try to keep memory ordering constraints on atomics as weak as possible,
// since memory fences or ordered memory accesses are likely to be a major
// performance cost for this code. All accesses to mStrong, mWeak, and mFlags
// explicitly relax memory ordering in some way.
//
// The only operations that are not memory_order_relaxed are reference count
// decrements. All reference count decrements are release operations. In
// addition, the final decrement leading the deallocation is followed by an
// acquire fence, which we can view informally as also turning it into an
// acquire operation. (See 29.8p4 [atomics.fences] for details. We could
// alternatively use acq_rel operations for all decrements. This is probably
// slower on most current (2016) hardware, especially on ARMv7, but that may
// not be true indefinitely.)
//
// This convention ensures that the second-to-last decrement synchronizes with
// (in the language of 1.10 in the C++ standard) the final decrement of a
// reference count. Since reference counts are only updated using atomic
// read-modify-write operations, this also extends to any earlier decrements.
// (See "release sequence" in 1.10.)
//
// Since all operations on an object happen before the corresponding reference
// count decrement, and all reference count decrements happen before the final
// one, we are guaranteed that all other object accesses happen before the
// object is destroyed.
#define INITIAL_STRONG_VALUE (1<<28)
// ---------------------------------------------------------------------------
class RefBase::weakref_impl : public RefBase::weakref_type
{
public:
std::atomic<int32_t> mStrong;
std::atomic<int32_t> mWeak;
RefBase* const mBase;
std::atomic<int32_t> mFlags;
#if !DEBUG_REFS
weakref_impl(RefBase* base)
: mStrong(INITIAL_STRONG_VALUE)
, mWeak(0)
, mBase(base)
, mFlags(0)
{
}
void addStrongRef(const void* /*id*/) { }
void removeStrongRef(const void* /*id*/) { }
void renameStrongRefId(const void* /*old_id*/, const void* /*new_id*/) { }
void addWeakRef(const void* /*id*/) { }
void removeWeakRef(const void* /*id*/) { }
void renameWeakRefId(const void* /*old_id*/, const void* /*new_id*/) { }
void printRefs() const { }
void trackMe(bool, bool) { }
#else
weakref_impl(RefBase* base)
: mStrong(INITIAL_STRONG_VALUE)
, mWeak(0)
, mBase(base)
, mFlags(0)
, mStrongRefs(NULL)
, mWeakRefs(NULL)
, mTrackEnabled(!!DEBUG_REFS_ENABLED_BY_DEFAULT)
, mRetain(false)
{
}
~weakref_impl()
{
bool dumpStack = false;
if (!mRetain && mStrongRefs != NULL) {
dumpStack = true;
ALOGE("Strong references remain:");
ref_entry* refs = mStrongRefs;
while (refs) {
char inc = refs->ref >= 0 ? '+' : '-';
ALOGD("\t%c ID %p (ref %d):", inc, refs->id, refs->ref);
#if DEBUG_REFS_CALLSTACK_ENABLED
refs->stack.log(LOG_TAG);
#endif
refs = refs->next;
}
}
if (!mRetain && mWeakRefs != NULL) {
dumpStack = true;
ALOGE("Weak references remain!");
ref_entry* refs = mWeakRefs;
while (refs) {
char inc = refs->ref >= 0 ? '+' : '-';
ALOGD("\t%c ID %p (ref %d):", inc, refs->id, refs->ref);
#if DEBUG_REFS_CALLSTACK_ENABLED
refs->stack.log(LOG_TAG);
#endif
refs = refs->next;
}
}
if (dumpStack) {
ALOGE("above errors at:");
CallStack stack(LOG_TAG);
}
}
void addStrongRef(const void* id) {
//ALOGD_IF(mTrackEnabled,
// "addStrongRef: RefBase=%p, id=%p", mBase, id);
addRef(&mStrongRefs, id, mStrong.load(std::memory_order_relaxed));
}
void removeStrongRef(const void* id) {
//ALOGD_IF(mTrackEnabled,
// "removeStrongRef: RefBase=%p, id=%p", mBase, id);
if (!mRetain) {
removeRef(&mStrongRefs, id);
} else {
addRef(&mStrongRefs, id, -mStrong.load(std::memory_order_relaxed));
}
}
void renameStrongRefId(const void* old_id, const void* new_id) {
//ALOGD_IF(mTrackEnabled,
// "renameStrongRefId: RefBase=%p, oid=%p, nid=%p",
// mBase, old_id, new_id);
renameRefsId(mStrongRefs, old_id, new_id);
}
void addWeakRef(const void* id) {
addRef(&mWeakRefs, id, mWeak.load(std::memory_order_relaxed));
}
void removeWeakRef(const void* id) {
if (!mRetain) {
removeRef(&mWeakRefs, id);
} else {
addRef(&mWeakRefs, id, -mWeak.load(std::memory_order_relaxed));
}
}
void renameWeakRefId(const void* old_id, const void* new_id) {
renameRefsId(mWeakRefs, old_id, new_id);
}
void trackMe(bool track, bool retain)
{
mTrackEnabled = track;
mRetain = retain;
}
void printRefs() const
{
String8 text;
{
Mutex::Autolock _l(mMutex);
char buf[128];
sprintf(buf, "Strong references on RefBase %p (weakref_type %p):\n", mBase, this);
text.append(buf);
printRefsLocked(&text, mStrongRefs);
sprintf(buf, "Weak references on RefBase %p (weakref_type %p):\n", mBase, this);
text.append(buf);
printRefsLocked(&text, mWeakRefs);
}
{
char name[100];
snprintf(name, 100, DEBUG_REFS_CALLSTACK_PATH "/%p.stack", this);
int rc = open(name, O_RDWR | O_CREAT | O_APPEND, 644);
if (rc >= 0) {
write(rc, text.string(), text.length());
close(rc);
ALOGD("STACK TRACE for %p saved in %s", this, name);
}
else ALOGE("FAILED TO PRINT STACK TRACE for %p in %s: %s", this,
name, strerror(errno));
}
}
private:
struct ref_entry
{
ref_entry* next;
const void* id;
#if DEBUG_REFS_CALLSTACK_ENABLED
CallStack stack;
#endif
int32_t ref;
};
void addRef(ref_entry** refs, const void* id, int32_t mRef)
{
if (mTrackEnabled) {
AutoMutex _l(mMutex);
ref_entry* ref = new ref_entry;
// Reference count at the time of the snapshot, but before the
// update. Positive value means we increment, negative--we
// decrement the reference count.
ref->ref = mRef;
ref->id = id;
#if DEBUG_REFS_CALLSTACK_ENABLED
ref->stack.update(2);
#endif
ref->next = *refs;
*refs = ref;
}
}
void removeRef(ref_entry** refs, const void* id)
{
if (mTrackEnabled) {
AutoMutex _l(mMutex);
ref_entry* const head = *refs;
ref_entry* ref = head;
while (ref != NULL) {
if (ref->id == id) {
*refs = ref->next;
delete ref;
return;
}
refs = &ref->next;
ref = *refs;
}
ALOGE("RefBase: removing id %p on RefBase %p"
"(weakref_type %p) that doesn't exist!",
id, mBase, this);
ref = head;
while (ref) {
char inc = ref->ref >= 0 ? '+' : '-';
ALOGD("\t%c ID %p (ref %d):", inc, ref->id, ref->ref);
ref = ref->next;
}
CallStack stack(LOG_TAG);
}
}
void renameRefsId(ref_entry* r, const void* old_id, const void* new_id)
{
if (mTrackEnabled) {
AutoMutex _l(mMutex);
ref_entry* ref = r;
while (ref != NULL) {
if (ref->id == old_id) {
ref->id = new_id;
}
ref = ref->next;
}
}
}
void printRefsLocked(String8* out, const ref_entry* refs) const
{
char buf[128];
while (refs) {
char inc = refs->ref >= 0 ? '+' : '-';
sprintf(buf, "\t%c ID %p (ref %d):\n",
inc, refs->id, refs->ref);
out->append(buf);
#if DEBUG_REFS_CALLSTACK_ENABLED
out->append(refs->stack.toString("\t\t"));
#else
out->append("\t\t(call stacks disabled)");
#endif
refs = refs->next;
}
}
mutable Mutex mMutex;
ref_entry* mStrongRefs;
ref_entry* mWeakRefs;
bool mTrackEnabled;
// Collect stack traces on addref and removeref, instead of deleting the stack references
// on removeref that match the address ones.
bool mRetain;
#endif
};
// ---------------------------------------------------------------------------
void RefBase::incStrong(const void* id) const
{
weakref_impl* const refs = mRefs;
refs->incWeak(id);
refs->addStrongRef(id);
const int32_t c = refs->mStrong.fetch_add(1, std::memory_order_relaxed);
ALOG_ASSERT(c > 0, "incStrong() called on %p after last strong ref", refs);
#if PRINT_REFS
ALOGD("incStrong of %p from %p: cnt=%d\n", this, id, c);
#endif
if (c != INITIAL_STRONG_VALUE) {
return;
}
int32_t old = refs->mStrong.fetch_sub(INITIAL_STRONG_VALUE,
std::memory_order_relaxed);
// A decStrong() must still happen after us.
ALOG_ASSERT(old > INITIAL_STRONG_VALUE, "0x%x too small", old);
refs->mBase->onFirstRef();
}
void RefBase::decStrong(const void* id) const
{
weakref_impl* const refs = mRefs;
refs->removeStrongRef(id);
const int32_t c = refs->mStrong.fetch_sub(1, std::memory_order_release);
#if PRINT_REFS
ALOGD("decStrong of %p from %p: cnt=%d\n", this, id, c);
#endif
ALOG_ASSERT(c >= 1, "decStrong() called on %p too many times", refs);
if (c == 1) {
std::atomic_thread_fence(std::memory_order_acquire);
refs->mBase->onLastStrongRef(id);
int32_t flags = refs->mFlags.load(std::memory_order_relaxed);
if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {
delete this;
// Since mStrong had been incremented, the destructor did not
// delete refs.
}
}
// Note that even with only strong reference operations, the thread
// deallocating this may not be the same as the thread deallocating refs.
// That's OK: all accesses to this happen before its deletion here,
// and all accesses to refs happen before its deletion in the final decWeak.
// The destructor can safely access mRefs because either it's deleting
// mRefs itself, or it's running entirely before the final mWeak decrement.
refs->decWeak(id);
}
void RefBase::forceIncStrong(const void* id) const
{
// Allows initial mStrong of 0 in addition to INITIAL_STRONG_VALUE.
// TODO: Better document assumptions.
weakref_impl* const refs = mRefs;
refs->incWeak(id);
refs->addStrongRef(id);
const int32_t c = refs->mStrong.fetch_add(1, std::memory_order_relaxed);
ALOG_ASSERT(c >= 0, "forceIncStrong called on %p after ref count underflow",
refs);
#if PRINT_REFS
ALOGD("forceIncStrong of %p from %p: cnt=%d\n", this, id, c);
#endif
switch (c) {
case INITIAL_STRONG_VALUE:
refs->mStrong.fetch_sub(INITIAL_STRONG_VALUE,
std::memory_order_relaxed);
// fall through...
case 0:
refs->mBase->onFirstRef();
}
}
int32_t RefBase::getStrongCount() const
{
// Debugging only; No memory ordering guarantees.
return mRefs->mStrong.load(std::memory_order_relaxed);
}
RefBase* RefBase::weakref_type::refBase() const
{
return static_cast<const weakref_impl*>(this)->mBase;
}
void RefBase::weakref_type::incWeak(const void* id)
{
weakref_impl* const impl = static_cast<weakref_impl*>(this);
impl->addWeakRef(id);
const int32_t c __unused = impl->mWeak.fetch_add(1,
std::memory_order_relaxed);
ALOG_ASSERT(c >= 0, "incWeak called on %p after last weak ref", this);
}
void RefBase::weakref_type::decWeak(const void* id)
{
weakref_impl* const impl = static_cast<weakref_impl*>(this);
impl->removeWeakRef(id);
const int32_t c = impl->mWeak.fetch_sub(1, std::memory_order_release);
ALOG_ASSERT(c >= 1, "decWeak called on %p too many times", this);
if (c != 1) return;
atomic_thread_fence(std::memory_order_acquire);
int32_t flags = impl->mFlags.load(std::memory_order_relaxed);
if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {
// This is the regular lifetime case. The object is destroyed
// when the last strong reference goes away. Since weakref_impl
// outlive the object, it is not destroyed in the dtor, and
// we'll have to do it here.
if (impl->mStrong.load(std::memory_order_relaxed)
== INITIAL_STRONG_VALUE) {
// Special case: we never had a strong reference, so we need to
// destroy the object now.
delete impl->mBase;
} else {
// ALOGV("Freeing refs %p of old RefBase %p\n", this, impl->mBase);
delete impl;
}
} else {
// This is the OBJECT_LIFETIME_WEAK case. The last weak-reference
// is gone, we can destroy the object.
impl->mBase->onLastWeakRef(id);
delete impl->mBase;
}
}
bool RefBase::weakref_type::attemptIncStrong(const void* id)
{
incWeak(id);
weakref_impl* const impl = static_cast<weakref_impl*>(this);
int32_t curCount = impl->mStrong.load(std::memory_order_relaxed);
ALOG_ASSERT(curCount >= 0,
"attemptIncStrong called on %p after underflow", this);
while (curCount > 0 && curCount != INITIAL_STRONG_VALUE) {
// we're in the easy/common case of promoting a weak-reference
// from an existing strong reference.
if (impl->mStrong.compare_exchange_weak(curCount, curCount+1,
std::memory_order_relaxed)) {
break;
}
// the strong count has changed on us, we need to re-assert our
// situation. curCount was updated by compare_exchange_weak.
}
if (curCount <= 0 || curCount == INITIAL_STRONG_VALUE) {
// we're now in the harder case of either:
// - there never was a strong reference on us
// - or, all strong references have been released
int32_t flags = impl->mFlags.load(std::memory_order_relaxed);
if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {
// this object has a "normal" life-time, i.e.: it gets destroyed
// when the last strong reference goes away
if (curCount <= 0) {
// the last strong-reference got released, the object cannot
// be revived.
decWeak(id);
return false;
}
// here, curCount == INITIAL_STRONG_VALUE, which means
// there never was a strong-reference, so we can try to
// promote this object; we need to do that atomically.
while (curCount > 0) {
if (impl->mStrong.compare_exchange_weak(curCount, curCount+1,
std::memory_order_relaxed)) {
break;
}
// the strong count has changed on us, we need to re-assert our
// situation (e.g.: another thread has inc/decStrong'ed us)
// curCount has been updated.
}
if (curCount <= 0) {
// promote() failed, some other thread destroyed us in the
// meantime (i.e.: strong count reached zero).
decWeak(id);
return false;
}
} else {
// this object has an "extended" life-time, i.e.: it can be
// revived from a weak-reference only.
// Ask the object's implementation if it agrees to be revived
if (!impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id)) {
// it didn't so give-up.
decWeak(id);
return false;
}
// grab a strong-reference, which is always safe due to the
// extended life-time.
curCount = impl->mStrong.fetch_add(1, std::memory_order_relaxed);
}
// If the strong reference count has already been incremented by
// someone else, the implementor of onIncStrongAttempted() is holding
// an unneeded reference. So call onLastStrongRef() here to remove it.
// (No, this is not pretty.) Note that we MUST NOT do this if we
// are in fact acquiring the first reference.
if (curCount > 0 && curCount < INITIAL_STRONG_VALUE) {
impl->mBase->onLastStrongRef(id);
}
}
impl->addStrongRef(id);
#if PRINT_REFS
ALOGD("attemptIncStrong of %p from %p: cnt=%d\n", this, id, curCount);
#endif
// curCount is the value of mStrong before we increment ed it.
// Now we need to fix-up the count if it was INITIAL_STRONG_VALUE.
// This must be done safely, i.e.: handle the case where several threads
// were here in attemptIncStrong().
// curCount > INITIAL_STRONG_VALUE is OK, and can happen if we're doing
// this in the middle of another incStrong. The subtraction is handled
// by the thread that started with INITIAL_STRONG_VALUE.
if (curCount == INITIAL_STRONG_VALUE) {
impl->mStrong.fetch_sub(INITIAL_STRONG_VALUE,
std::memory_order_relaxed);
}
return true;
}
bool RefBase::weakref_type::attemptIncWeak(const void* id)
{
weakref_impl* const impl = static_cast<weakref_impl*>(this);
int32_t curCount = impl->mWeak.load(std::memory_order_relaxed);
ALOG_ASSERT(curCount >= 0, "attemptIncWeak called on %p after underflow",
this);
while (curCount > 0) {
if (impl->mWeak.compare_exchange_weak(curCount, curCount+1,
std::memory_order_relaxed)) {
break;
}
// curCount has been updated.
}
if (curCount > 0) {
impl->addWeakRef(id);
}
return curCount > 0;
}
int32_t RefBase::weakref_type::getWeakCount() const
{
// Debug only!
return static_cast<const weakref_impl*>(this)->mWeak
.load(std::memory_order_relaxed);
}
void RefBase::weakref_type::printRefs() const
{
static_cast<const weakref_impl*>(this)->printRefs();
}
void RefBase::weakref_type::trackMe(bool enable, bool retain)
{
static_cast<weakref_impl*>(this)->trackMe(enable, retain);
}
RefBase::weakref_type* RefBase::createWeak(const void* id) const
{
mRefs->incWeak(id);
return mRefs;
}
RefBase::weakref_type* RefBase::getWeakRefs() const
{
return mRefs;
}
RefBase::RefBase()
: mRefs(new weakref_impl(this))
{
}
RefBase::~RefBase()
{
if (mRefs->mStrong.load(std::memory_order_relaxed)
== INITIAL_STRONG_VALUE) {
// we never acquired a strong (and/or weak) reference on this object.
delete mRefs;
} else {
// life-time of this object is extended to WEAK, in
// which case weakref_impl doesn't out-live the object and we
// can free it now.
int32_t flags = mRefs->mFlags.load(std::memory_order_relaxed);
if ((flags & OBJECT_LIFETIME_MASK) != OBJECT_LIFETIME_STRONG) {
// It's possible that the weak count is not 0 if the object
// re-acquired a weak reference in its destructor
if (mRefs->mWeak.load(std::memory_order_relaxed) == 0) {
delete mRefs;
}
}
}
// for debugging purposes, clear this.
const_cast<weakref_impl*&>(mRefs) = NULL;
}
void RefBase::extendObjectLifetime(int32_t mode)
{
// Must be happens-before ordered with respect to construction or any
// operation that could destroy the object.
mRefs->mFlags.fetch_or(mode, std::memory_order_relaxed);
}
void RefBase::onFirstRef()
{
}
void RefBase::onLastStrongRef(const void* /*id*/)
{
}
bool RefBase::onIncStrongAttempted(uint32_t flags, const void* /*id*/)
{
return (flags&FIRST_INC_STRONG) ? true : false;
}
void RefBase::onLastWeakRef(const void* /*id*/)
{
}
// ---------------------------------------------------------------------------
#if DEBUG_REFS
void RefBase::renameRefs(size_t n, const ReferenceRenamer& renamer) {
for (size_t i=0 ; i<n ; i++) {
renamer(i);
}
}
#else
void RefBase::renameRefs(size_t /*n*/, const ReferenceRenamer& /*renamer*/) { }
#endif
void RefBase::renameRefId(weakref_type* ref,
const void* old_id, const void* new_id) {
weakref_impl* const impl = static_cast<weakref_impl*>(ref);
impl->renameStrongRefId(old_id, new_id);
impl->renameWeakRefId(old_id, new_id);
}
void RefBase::renameRefId(RefBase* ref,
const void* old_id, const void* new_id) {
ref->mRefs->renameStrongRefId(old_id, new_id);
ref->mRefs->renameWeakRefId(old_id, new_id);
}
}; // namespace android
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