fix a couple race-conditions in RefBase::promote()
Bug: 8390295 Change-Id: I7a48e3bf5b213cc1da2b8e844c6bb37ee24cb047
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@ -441,39 +441,68 @@ bool RefBase::weakref_type::attemptIncStrong(const void* id)
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incWeak(id);
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weakref_impl* const impl = static_cast<weakref_impl*>(this);
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int32_t curCount = impl->mStrong;
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ALOG_ASSERT(curCount >= 0, "attemptIncStrong called on %p after underflow",
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this);
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ALOG_ASSERT(curCount >= 0,
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"attemptIncStrong called on %p after underflow", this);
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while (curCount > 0 && curCount != INITIAL_STRONG_VALUE) {
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// we're in the easy/common case of promoting a weak-reference
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// from an existing strong reference.
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if (android_atomic_cmpxchg(curCount, curCount+1, &impl->mStrong) == 0) {
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break;
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}
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// the strong count has changed on us, we need to re-assert our
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// situation.
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curCount = impl->mStrong;
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}
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if (curCount <= 0 || curCount == INITIAL_STRONG_VALUE) {
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bool allow;
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if (curCount == INITIAL_STRONG_VALUE) {
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// Attempting to acquire first strong reference... this is allowed
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// if the object does NOT have a longer lifetime (meaning the
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// implementation doesn't need to see this), or if the implementation
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// allows it to happen.
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allow = (impl->mFlags&OBJECT_LIFETIME_WEAK) != OBJECT_LIFETIME_WEAK
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|| impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id);
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// we're now in the harder case of either:
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// - there never was a strong reference on us
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// - or, all strong references have been released
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if ((impl->mFlags&OBJECT_LIFETIME_WEAK) == OBJECT_LIFETIME_STRONG) {
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// this object has a "normal" life-time, i.e.: it gets destroyed
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// when the last strong reference goes away
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if (curCount <= 0) {
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// the last strong-reference got released, the object cannot
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// be revived.
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decWeak(id);
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return false;
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}
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// here, curCount == INITIAL_STRONG_VALUE, which means
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// there never was a strong-reference, so we can try to
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// promote this object; we need to do that atomically.
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while (curCount > 0) {
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if (android_atomic_cmpxchg(curCount, curCount + 1,
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&impl->mStrong) == 0) {
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break;
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}
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// the strong count has changed on us, we need to re-assert our
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// situation (e.g.: another thread has inc/decStrong'ed us)
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curCount = impl->mStrong;
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}
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if (curCount <= 0) {
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// promote() failed, some other thread destroyed us in the
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// meantime (i.e.: strong count reached zero).
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decWeak(id);
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return false;
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}
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} else {
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// Attempting to revive the object... this is allowed
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// if the object DOES have a longer lifetime (so we can safely
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// call the object with only a weak ref) and the implementation
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// allows it to happen.
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allow = (impl->mFlags&OBJECT_LIFETIME_WEAK) == OBJECT_LIFETIME_WEAK
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&& impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id);
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// this object has an "extended" life-time, i.e.: it can be
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// revived from a weak-reference only.
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// Ask the object's implementation if it agrees to be revived
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if (!impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id)) {
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// it didn't so give-up.
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decWeak(id);
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return false;
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}
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// grab a strong-reference, which is always safe due to the
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// extended life-time.
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curCount = android_atomic_inc(&impl->mStrong);
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}
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if (!allow) {
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decWeak(id);
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return false;
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}
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curCount = android_atomic_inc(&impl->mStrong);
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// If the strong reference count has already been incremented by
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// someone else, the implementor of onIncStrongAttempted() is holding
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@ -491,11 +520,23 @@ bool RefBase::weakref_type::attemptIncStrong(const void* id)
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ALOGD("attemptIncStrong of %p from %p: cnt=%d\n", this, id, curCount);
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#endif
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if (curCount == INITIAL_STRONG_VALUE) {
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android_atomic_add(-INITIAL_STRONG_VALUE, &impl->mStrong);
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impl->mBase->onFirstRef();
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// now we need to fix-up the count if it was INITIAL_STRONG_VALUE
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// this must be done safely, i.e.: handle the case where several threads
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// were here in attemptIncStrong().
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curCount = impl->mStrong;
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while (curCount >= INITIAL_STRONG_VALUE) {
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ALOG_ASSERT(curCount > INITIAL_STRONG_VALUE,
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"attemptIncStrong in %p underflowed to INITIAL_STRONG_VALUE",
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this);
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if (android_atomic_cmpxchg(curCount, curCount-INITIAL_STRONG_VALUE,
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&impl->mStrong) == 0) {
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break;
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}
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// the strong-count changed on us, we need to re-assert the situation,
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// for e.g.: it's possible the fix-up happened in another thread.
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curCount = impl->mStrong;
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}
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return true;
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}
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