Merge "Make RefBase more robust and debuggable" (daac359b) · Commits · e / os / android_system_core

include/utils/RefBase.h

+25 −12

Original line number	Diff line number	Diff line
		@@ -105,16 +105,14 @@

		// Other more specific restrictions for wp<> and sp<>:

		// Constructing a strong or weak pointer to "this" in its constructors is almost
		// always wrong. In the case of strong pointers. it is always wrong with RefBase
		// because the onFirstRef() callback will be mode on an incompletely constructed
		// object. In either case, it is wrong if such a pointer does not outlive the
		// constructor, since destruction of the smart pointer will attempt to destroy the
		// object before construction is finished, normally resulting in a pointer to a
		// destroyed object being returned from a new expression.

		// In the case of weak pointers, this occurs because an object that has never been
		// referenced by a strong pointer is destroyed when the last weak pointer disappears.
		// Do not construct a strong pointer to "this" in an object's constructor.
		// The onFirstRef() callback would be made on an incompletely constructed
		// object.
		// Construction of a weak pointer to "this" in an object's constructor is also
		// discouraged. But the implementation was recently changed so that, in the
		// absence of extendObjectLifetime() calls, weak pointers no longer impact
		// object lifetime, and hence this no longer risks premature deallocation,
		// and hence usually works correctly.

		// Such strong or weak pointers can be safely created in the RefBase onFirstRef()
		// callback.
		@@ -126,8 +124,23 @@
		// is a longer-lived sp<>, why not use an sp<> directly?) A wp<> should only be
		// dereferenced by using promote().

		// Any object inheriting from RefBase should always be destroyed as the result
		// of a reference count decrement, not via any other means. Such objects
		// should never be stack allocated, or appear directly as data members in other
		// objects. Objects inheriting from RefBase should have their strong reference
		// count incremented as soon as possible after construction. Usually this
		// will be done via construction of an sp<> to the object, but may instead
		// involve other means of calling RefBase::incStrong().
		// Explicitly deleting or otherwise destroying a RefBase object with outstanding
		// wp<> or sp<> pointers to it will result in heap corruption.
		// wp<> or sp<> pointers to it will result in an abort or heap corruption.

		// It is particularly important not to mix sp<> and direct storage management
		// since the sp from raw pointer constructor is implicit. Thus if a RefBase-
		// -derived object of type T is managed without ever incrementing its strong
		// count, and accidentally passed to f(sp<T>), a strong pointer to the object
		// will be temporarily constructed and destroyed, prematurely deallocating the
		// object, and resulting in heap corruption. None of this would be easily
		// visible in the source.

		// Extra Features:

		@@ -144,7 +157,7 @@
		// events, as well as some debugging facilities.

		// Debugging support can be enabled by turning on DEBUG_REFS in RefBase.cpp.
		// Otherwise essentially no checking is provided.
		// Otherwise little checking is provided.

		// Thread safety:

libutils/RefBase.cpp

+56 −32

Original line number	Diff line number	Diff line
		@@ -84,15 +84,16 @@ namespace android {
		//
		// 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.
		// 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
		@@ -126,6 +127,19 @@ namespace android {

		#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
		@@ -421,15 +435,15 @@ void RefBase::decStrong(const void* id) const
		#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);
		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;
		// Since mStrong had been incremented, the destructor did not
		// delete refs.
		// The destructor does not delete refs in this case.
		}
		}
		// Note that even with only strong reference operations, the thread
		@@ -492,7 +506,8 @@ 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);
		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);

		@@ -500,13 +515,19 @@ void RefBase::weakref_type::decWeak(const void* id)
		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
		// 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) {
		// Special case: we never had a strong reference, so we need to
		// destroy the object now.
		delete impl->mBase;
		// 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;
		@@ -675,25 +696,28 @@ RefBase::RefBase()

		RefBase::~RefBase()
		{
		if (mRefs->mStrong.load(std::memory_order_relaxed)
		== INITIAL_STRONG_VALUE) {
		// We never acquired a strong reference on this object.
		// We assume there are no outstanding weak references.
		delete mRefs;
		} else {
		// life-time of this object is extended to WEAK, in
		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.
		int32_t flags = mRefs->mFlags.load(std::memory_order_relaxed);
		if ((flags & OBJECT_LIFETIME_MASK) != OBJECT_LIFETIME_STRONG) {
		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.
		LOG_ALWAYS_FATAL_IF(mRefs->mWeak.load() != 0,
		"RefBase: Explicit destruction with non-zero weak "
		"reference count");
		// TODO: Always report if we get here. Currently MediaMetadataRetriever
		// C++ objects are inconsistently managed and sometimes get here.
		// There may be other cases, but we believe they should all be fixed.
		delete mRefs;
		}
		}
		// for debugging purposes, clear this.
		// For debugging purposes, clear mRefs. Ineffective against outstanding wp's.
		const_cast<weakref_impl*&>(mRefs) = NULL;
		}

libutils/tests/RefBase_test.cpp

+95 −21

Original line number	Diff line number	Diff line
		@@ -87,7 +87,7 @@ TEST(RefBase, WeakCopies) {
		EXPECT_EQ(1, foo->getWeakRefs()->getWeakCount());
		ASSERT_FALSE(isDeleted) << "deleted too early! still has a reference!";
		wp1 = nullptr;
		ASSERT_TRUE(isDeleted) << "foo2 was leaked!";
		ASSERT_FALSE(isDeleted) << "Deletion on wp destruction should no longer occur";
		}


		@@ -121,8 +121,33 @@ static inline void waitFor(bool val) {

		cpu_set_t otherCpus;

		// Divide the cpus we're allowed to run on into myCpus and otherCpus.
		// Set origCpus to the processors we were originally allowed to run on.
		// Return false if origCpus doesn't include at least processors 0 and 1.
		static bool setExclusiveCpus(cpu_set_t* origCpus /* out */,
		cpu_set_t* myCpus /* out /, cpu_set_t otherCpus) {
		if (sched_getaffinity(0, sizeof(cpu_set_t), origCpus) != 0) {
		return false;
		}
		if (!CPU_ISSET(0, origCpus) \|\| !CPU_ISSET(1, origCpus)) {
		return false;
		}
		CPU_ZERO(myCpus);
		CPU_ZERO(otherCpus);
		CPU_OR(myCpus, myCpus, origCpus);
		CPU_OR(otherCpus, otherCpus, origCpus);
		for (unsigned i = 0; i < CPU_SETSIZE; ++i) {
		// I get the even cores, the other thread gets the odd ones.
		if (i & 1) {
		CPU_CLR(i, myCpus);
		} else {
		CPU_CLR(i, otherCpus);
		}
		}
		return true;
		}

		static void visit2AndRemove() {
		EXPECT_TRUE(CPU_ISSET(1, &otherCpus));
		if (sched_setaffinity(0, sizeof(cpu_set_t), &otherCpus) != 0) {
		FAIL() << "setaffinity returned:" << errno;
		}
		@@ -139,27 +164,10 @@ TEST(RefBase, RacingDestructors) {
		cpu_set_t myCpus;
		// Restrict us and the helper thread to disjoint cpu sets.
		// This prevents us from getting scheduled against each other,
		// which would be atrociously slow. We fail if that's impossible.
		if (sched_getaffinity(0, sizeof(cpu_set_t), &origCpus) != 0) {
		FAIL();
		}
		EXPECT_TRUE(CPU_ISSET(0, &origCpus));
		if (CPU_ISSET(1, &origCpus)) {
		CPU_ZERO(&myCpus);
		CPU_ZERO(&otherCpus);
		CPU_OR(&myCpus, &myCpus, &origCpus);
		CPU_OR(&otherCpus, &otherCpus, &origCpus);
		for (unsigned i = 0; i < CPU_SETSIZE; ++i) {
		// I get the even cores, the other thread gets the odd ones.
		if (i & 1) {
		CPU_CLR(i, &myCpus);
		} else {
		CPU_CLR(i, &otherCpus);
		}
		}
		// which would be atrociously slow.
		if (setExclusiveCpus(&origCpus, &myCpus, &otherCpus)) {
		std::thread t(visit2AndRemove);
		std::atomic<int> deleteCount(0);
		EXPECT_TRUE(CPU_ISSET(0, &myCpus));
		if (sched_setaffinity(0, sizeof(cpu_set_t), &myCpus) != 0) {
		FAIL() << "setaffinity returned:" << errno;
		}
		@@ -182,3 +190,69 @@ TEST(RefBase, RacingDestructors) {
		ASSERT_EQ(NITERS, deleteCount) << "Deletions missed!";
		} // Otherwise this is slow and probably pointless on a uniprocessor.
		}

		static wp<Bar> wpBuffer;
		static std::atomic<bool> wpBufferFull(false);

		// Wait until wpBufferFull has value val.
		static inline void wpWaitFor(bool val) {
		while (wpBufferFull != val) {}
		}

		static void visit3AndRemove() {
		if (sched_setaffinity(0, sizeof(cpu_set_t), &otherCpus) != 0) {
		FAIL() << "setaffinity returned:" << errno;
		}
		for (int i = 0; i < NITERS; ++i) {
		wpWaitFor(true);
		{
		sp<Bar> sp1 = wpBuffer.promote();
		// We implicitly check that sp1 != NULL
		sp1->mVisited2 = true;
		}
		wpBuffer = nullptr;
		wpBufferFull = false;
		}
		}

		TEST(RefBase, RacingPromotions) {
		cpu_set_t origCpus;
		cpu_set_t myCpus;
		// Restrict us and the helper thread to disjoint cpu sets.
		// This prevents us from getting scheduled against each other,
		// which would be atrociously slow.
		if (setExclusiveCpus(&origCpus, &myCpus, &otherCpus)) {
		std::thread t(visit3AndRemove);
		std::atomic<int> deleteCount(0);
		if (sched_setaffinity(0, sizeof(cpu_set_t), &myCpus) != 0) {
		FAIL() << "setaffinity returned:" << errno;
		}
		for (int i = 0; i < NITERS; ++i) {
		Bar* bar = new Bar(&deleteCount);
		wp<Bar> wp1(bar);
		bar->mVisited1 = true;
		if (i % (NITERS / 10) == 0) {
		// Do this rarely, since it generates a log message.
		wp1 = nullptr; // No longer destroys the object.
		wp1 = bar;
		}
		wpBuffer = wp1;
		ASSERT_EQ(bar->getWeakRefs()->getWeakCount(), 2);
		wpBufferFull = true;
		// Promotion races with that in visit3AndRemove.
		// This may or may not succeed, but it shouldn't interfere with
		// the concurrent one.
		sp<Bar> sp1 = wp1.promote();
		wpWaitFor(false); // Waits for other thread to drop strong pointer.
		sp1 = nullptr;
		// No strong pointers here.
		sp1 = wp1.promote();
		ASSERT_EQ(sp1.get(), nullptr) << "Dead wp promotion succeeded!";
		}
		t.join();
		if (sched_setaffinity(0, sizeof(cpu_set_t), &origCpus) != 0) {
		FAIL();
		}
		ASSERT_EQ(NITERS, deleteCount) << "Deletions missed!";
		} // Otherwise this is slow and probably pointless on a uniprocessor.
		}