1berry/qtbase
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

QHash: fix thread race around references and detaching

Browse Source 
If we detach from a shared hash while holding a reference to a key from
said shared hash then there is no guarantee for how long the reference
is valid (given a multi-thread environment).

Pick-to: 6.2
Change-Id: Ifb610753d24faca63e2c0eb8836c78d55a229001
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
This commit is contained in:
Mårten Nordheim 2021-10-26 15:04:19 +02:00 committed by Marc Mutz
parent a92619d950
commit 8f8775adf3
2 changed files with 141 additions and 36 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

121
src/corelib/tools/qhash.h
View File

@ -888,9 +888,10 @@ public:
{
if (isEmpty()) // prevents detaching shared null
return false;
detach();
auto it = d->find(key);
detach();
it = d->detachedIterator(it);
if (it.isUnused())
return false;
d->erase(it);
@ -905,9 +906,10 @@ public:
{
if (isEmpty()) // prevents detaching shared null
return T();
detach();
auto it = d->find(key);
detach();
it = d->detachedIterator(it);
if (it.isUnused())
return T();
T value = it.node()->takeValue();
@ -986,6 +988,7 @@ public:
T &operator[](const Key &key)
{
const auto copy = isDetached() ? QHash() : *this; // keep 'key' alive across the detach
detach();
auto result = d->findOrInsert(key);
Q_ASSERT(!result.it.atEnd());
@ -1178,8 +1181,9 @@ public:
{
if (isEmpty()) // prevents detaching shared null
return end();
detach();
auto it = d->find(key);
detach();
it = d->detachedIterator(it);
if (it.isUnused())
it = d->end();
return iterator(it);
@ -1227,14 +1231,15 @@ public:
template <typename ...Args>
iterator emplace(Key &&key, Args &&... args)
{
if (isDetached()) {
if (d->shouldGrow()) // Construct the value now so that no dangling references are used
return emplace_helper(std::move(key), T(std::forward<Args>(args)...));
return emplace_helper(std::move(key), std::forward<Args>(args)...);
}
// else: we must detach
const auto copy = *this; // keep 'args' alive across the detach/growth
detach();
auto result = d->findOrInsert(key);
if (!result.initialized)
Node::createInPlace(result.it.node(), std::move(key), std::forward<Args>(args)...);
else
result.it.node()->emplaceValue(std::forward<Args>(args)...);
return iterator(result.it);
return emplace_helper(std::move(key), std::forward<Args>(args)...);
}
float load_factor() const noexcept { return d ? d->loadFactor() : 0; }
@ -1243,6 +1248,18 @@ public:
static size_t max_bucket_count() noexcept { return QHashPrivate::GrowthPolicy::maxNumBuckets(); }
inline bool empty() const noexcept { return isEmpty(); }
private:
template <typename ...Args>
iterator emplace_helper(Key &&key, Args &&... args)
{
auto result = d->findOrInsert(key);
if (!result.initialized)
Node::createInPlace(result.it.node(), std::move(key), std::forward<Args>(args)...);
else
result.it.node()->emplaceValue(std::forward<Args>(args)...);
return iterator(result.it);
}
};
@ -1416,9 +1433,10 @@ public:
{
if (isEmpty()) // prevents detaching shared null
return 0;
detach();
auto it = d->find(key);
detach();
it = d->detachedIterator(it);
if (it.isUnused())
return 0;
qsizetype n = Node::freeChain(it.node());
@ -1436,9 +1454,10 @@ public:
{
if (isEmpty()) // prevents detaching shared null
return T();
detach();
auto it = d->find(key);
detach();
it = d->detachedIterator(it);
if (it.isUnused())
return T();
Chain *e = it.node()->value;
@ -1524,6 +1543,7 @@ public:
T &operator[](const Key &key)
{
const auto copy = isDetached() ? QMultiHash() : *this; // keep 'key' alive across the detach
detach();
auto result = d->findOrInsert(key);
Q_ASSERT(!result.it.atEnd());
@ -1784,8 +1804,10 @@ public:
{
if (isEmpty())
return end();
detach();
auto it = d->find(key);
detach();
it = d->detachedIterator(it);
if (it.isUnused())
it = d->end();
return iterator(it);
@ -1817,15 +1839,15 @@ public:
template <typename ...Args>
iterator emplace(Key &&key, Args &&... args)
{
if (isDetached()) {
if (d->shouldGrow()) // Construct the value now so that no dangling references are used
return emplace_helper(std::move(key), T(std::forward<Args>(args)...));
return emplace_helper(std::move(key), std::forward<Args>(args)...);
}
// else: we must detach
const auto copy = *this; // keep 'args' alive across the detach/growth
detach();
auto result = d->findOrInsert(key);
if (!result.initialized)
Node::createInPlace(result.it.node(), std::move(key), std::forward<Args>(args)...);
else
result.it.node()->insertMulti(std::forward<Args>(args)...);
++m_size;
return iterator(result.it);
return emplace_helper(std::move(key), std::forward<Args>(args)...);
}
@ -1850,16 +1872,15 @@ public:
template <typename ...Args>
iterator emplaceReplace(Key &&key, Args &&... args)
{
detach();
auto result = d->findOrInsert(key);
if (!result.initialized) {
++m_size;
Node::createInPlace(result.it.node(), std::move(key), std::forward<Args>(args)...);
} else {
result.it.node()->emplaceValue(std::forward<Args>(args)...);
if (isDetached()) {
if (d->shouldGrow()) // Construct the value now so that no dangling references are used
return emplaceReplace_helper(std::move(key), T(std::forward<Args>(args)...));
return emplaceReplace_helper(std::move(key), std::forward<Args>(args)...);
}
return iterator(result.it);
// else: we must detach
const auto copy = *this; // keep 'args' alive across the detach/growth
detach();
return emplaceReplace_helper(std::move(key), std::forward<Args>(args)...);
}
inline QMultiHash &operator+=(const QMultiHash &other)
@ -1881,9 +1902,10 @@ public:
{
if (isEmpty()) // prevents detaching shared null
return 0;
detach();
auto it = d->find(key);
detach();
it = d->detachedIterator(it);
if (it.isUnused())
return 0;
qsizetype n = 0;
@ -1944,6 +1966,7 @@ public:
{
if (isEmpty())
return end();
const auto copy = isDetached() ? QMultiHash() : *this; // keep 'key'/'value' alive across the detach
detach();
auto it = constFind(key, value);
return iterator(it.i, it.e);
@ -2001,6 +2024,7 @@ public:
QPair<iterator, iterator> equal_range(const Key &key)
{
const auto copy = isDetached() ? QMultiHash() : *this; // keep 'key' alive across the detach
detach();
auto pair = qAsConst(*this).equal_range(key);
return qMakePair(iterator(pair.first.i), iterator(pair.second.i));
@ -2031,6 +2055,31 @@ private:
delete d;
d = dd;
}
template<typename... Args>
iterator emplace_helper(Key &&key, Args &&...args)
{
auto result = d->findOrInsert(key);
if (!result.initialized)
Node::createInPlace(result.it.node(), std::move(key), std::forward<Args>(args)...);
else
result.it.node()->insertMulti(std::forward<Args>(args)...);
++m_size;
return iterator(result.it);
}
template<typename... Args>
iterator emplaceReplace_helper(Key &&key, Args &&...args)
{
auto result = d->findOrInsert(key);
if (!result.initialized) {
Node::createInPlace(result.it.node(), std::move(key), std::forward<Args>(args)...);
++m_size;
} else {
result.it.node()->emplaceValue(std::forward<Args>(args)...);
}
return iterator(result.it);
}
};
Q_DECLARE_ASSOCIATIVE_FORWARD_ITERATOR(Hash)

56
tests/auto/corelib/tools/qhash/tst_qhash.cpp
View File

@ -36,6 +36,8 @@
#include <unordered_set>
#include <string>
#include <qsemaphore.h>
class tst_QHash : public QObject
{
Q_OBJECT
@ -97,6 +99,8 @@ private slots:
void reserveShared();
void QTBUG98265();
void detachAndReferences();
};
struct IdentityTracker {
@ -2627,5 +2631,57 @@ void tst_QHash::QTBUG98265()
QVERIFY(a != b);
}
/*
Calling functions which take a const-ref argument for a key with a reference
to a key inside the hash itself should keep the key valid as long as it is
needed. If not users may get hard-to-debug races where CoW should've
shielded them.
*/
void tst_QHash::detachAndReferences()
{
#if !QT_CONFIG(cxx11_future)
QSKIP("This test requires cxx11_future")
#else
// Repeat a few times because it's not a guarantee
for (int i = 0; i < 50; ++i) {
QHash<char, char> hash;
hash.insert('a', 'a');
hash.insert('b', 'a');
hash.insert('c', 'a');
hash.insert('d', 'a');
hash.insert('e', 'a');
hash.insert('f', 'a');
hash.insert('g', 'a');
QSemaphore sem;
QSemaphore sem2;
std::thread th([&sem, &sem2, hash]() mutable {
sem.release();
sem2.acquire();
hash.reserve(100); // [2]: ...then this rehashes directly, without detaching
});
// The key is a reference to an entry in the hash. If we were already
// detached then no problem occurs! The problem happens because _after_
// we detach but before using the key the other thread resizes and
// rehashes, leaving our const-ref dangling.
auto it = hash.constBegin();
const auto &key = it.key(); // [3]: leaving our const-refs dangling
auto kCopy = key;
const auto &value = it.value();
auto vCopy = value;
sem2.release();
sem.acquire();
hash.insert(key, value); // [1]: this detaches first...
th.join();
QCOMPARE(hash.size(), 7);
QVERIFY(hash.contains(kCopy));
QCOMPARE(hash.value(kCopy), vCopy);
}
#endif
}
QTEST_APPLESS_MAIN(tst_QHash)
#include "tst_qhash.moc"