/* * 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 <memory> #include <android-base/macros.h> #include <utils/RefBase.h> #include <utils/CallStack.h> #include <utils/Mutex.h> #ifndef __unused #define __unused __attribute__((__unused__)) #endif // Compile with refcounting debugging enabled. #define DEBUG_REFS 0 // The following three are ignored unless DEBUG_REFS is set. // 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 // Continue after logging a stack trace if ~RefBase discovers that reference // count has never been incremented. Normally we conspicuously crash in that // case. #define DEBUG_REFBASE_DESTRUCTION 1 // --------------------------------------------------------------------------- namespace android { // Observations, invariants, etc: // By default, obects are destroyed when the last strong reference disappears // or, if the object never had a strong reference, when the last weak reference // disappears. // // 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. // // 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. // // 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 holds all the information, including both reference counts, // required to perform wp<> operations. Thus these can continue to be performed // after the RefBase object has been destroyed. // // A weakref_impl is allocated as the value of mRefs in a RefBase object on // construction. // In the OBJECT_LIFETIME_STRONG case, it is normally deallocated in decWeak, // and hence lives as long as the last weak reference. (It can also be // deallocated in the RefBase destructor iff the strong reference count was // never incremented and the weak count is zero, e.g. if the RefBase object is // explicitly destroyed without decrementing the strong count. This should be // avoided.) In this case, the RefBase destructor should be invoked from // decStrong. // In the OBJECT_LIFETIME_WEAK case, the weakref_impl is always deallocated in // 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) #define MAX_COUNT 0xfffff // Test whether the argument is a clearly invalid strong reference count. // Used only for error checking on the value before an atomic decrement. // Intended to be very cheap. // Note that we cannot just check for excess decrements by comparing to zero // since the object would be deallocated before that. #define BAD_STRONG(c) \ ((c) == 0 || ((c) & (~(MAX_COUNT | INITIAL_STRONG_VALUE))) != 0) // Same for weak counts. #define BAD_WEAK(c) ((c) == 0 || ((c) & (~MAX_COUNT)) != 0) // --------------------------------------------------------------------------- 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 explicit 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 CallStack::logStack(LOG_TAG, refs->stack.get()); #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 CallStack::logStack(LOG_TAG, refs->stack.get()); #endif refs = refs->next; } } if (dumpStack) { ALOGE("above errors at:"); CallStack::logStack(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]; snprintf(buf, sizeof(buf), "Strong references on RefBase %p (weakref_type %p):\n", mBase, this); text.append(buf); printRefsLocked(&text, mStrongRefs); snprintf(buf, sizeof(buf), "Weak references on RefBase %p (weakref_type %p):\n", mBase, this); text.append(buf); printRefsLocked(&text, mWeakRefs); } { char name[100]; snprintf(name, sizeof(name), DEBUG_REFS_CALLSTACK_PATH "/%p.stack", this); int rc = open(name, O_RDWR | O_CREAT | O_APPEND, 644); if (rc >= 0) { (void)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::CallStackUPtr 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 = CallStack::getCurrent(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::logStack(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 ? '+' : '-'; snprintf(buf, sizeof(buf), "\t%c ID %p (ref %d):\n", inc, refs->id, refs->ref); out->append(buf); #if DEBUG_REFS_CALLSTACK_ENABLED out->append(CallStack::stackToString("\t\t", refs->stack.get())); #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 __unused = 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 LOG_ALWAYS_FATAL_IF(BAD_STRONG(c), "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; // The destructor does not delete refs in this case. } } // 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. // // Since we're doing atomic loads of `flags`, the static analyzer assumes // they can change between `delete this;` and `refs->decWeak(id);`. This is // not the case. The analyzer may become more okay with this patten when // https://bugs.llvm.org/show_bug.cgi?id=34365 gets resolved. NOLINTNEXTLINE 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); FALLTHROUGH_INTENDED; 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); LOG_ALWAYS_FATAL_IF(BAD_WEAK(c), "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 // outlives 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) { // Decrementing a weak count to zero when object never had a strong // reference. We assume it acquired a weak reference early, e.g. // in the constructor, and will eventually be properly destroyed, // usually via incrementing and decrementing the strong count. // Thus we no longer do anything here. We log this case, since it // seems to be extremely rare, and should not normally occur. We // used to deallocate mBase here, so this may now indicate a leak. ALOGW("RefBase: Object at %p lost last weak reference " "before it had a strong reference", 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 incremented 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() { int32_t flags = mRefs->mFlags.load(std::memory_order_relaxed); // 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. if ((flags & OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_WEAK) { // 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; } } else if (mRefs->mStrong.load(std::memory_order_relaxed) == INITIAL_STRONG_VALUE) { // We never acquired a strong reference on this object. #if DEBUG_REFBASE_DESTRUCTION // Treating this as fatal is prone to causing boot loops. For debugging, it's // better to treat as non-fatal. ALOGD("RefBase: Explicit destruction, weak count = %d (in %p)", mRefs->mWeak.load(), this); CallStack::logStack(LOG_TAG); #else LOG_ALWAYS_FATAL("RefBase: Explicit destruction, weak count = %d", mRefs->mWeak.load()); #endif } // For debugging purposes, clear mRefs. Ineffective against outstanding wp's. const_cast<weakref_impl*&>(mRefs) = nullptr; } 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