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LibCore: Fix deadlock in SharedSingleProducerCircularQueue

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This deadlock would incorrectly change the queue from almost empty to
full on dequeue because someone else could empty the queue after we had
checked its non-emptyness. The test would deadlock on this, which
doesn't happen anymore.
This commit is contained in:
kleines Filmröllchen 2022-08-22 19:23:01 +02:00 • committed by Andreas Kling
parent be9d3f9aa4
commit 4355a01455
1 changed files with 6 additions and 5 deletions
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11
Userland/Libraries/LibCore/SharedCircularQueue.h
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@ -99,7 +99,7 @@ public:
return QueueStatus::Full; return QueueStatus::Full;
auto our_tail = m_queue->m_queue->m_tail.load() % Size; auto our_tail = m_queue->m_queue->m_tail.load() % Size;
m_queue->m_queue->m_data[our_tail] = to_insert; m_queue->m_queue->m_data[our_tail] = to_insert;
++m_queue->m_queue->m_tail; m_queue->m_queue->m_tail.fetch_add(1);
return {}; return {};
} }
@ -129,14 +129,15 @@ public:
{ {
VERIFY(!m_queue.is_null()); VERIFY(!m_queue.is_null());
while (true) { while (true) {
// The >= is not strictly necessary, but it feels safer :^)
if (head() >= m_queue->m_queue->m_tail.load())
return QueueStatus::Empty;
// This CAS only succeeds if nobody is currently dequeuing. // This CAS only succeeds if nobody is currently dequeuing.
auto size_max = NumericLimits<size_t>::max(); auto size_max = NumericLimits<size_t>::max();
if (m_queue->m_queue->m_head_protector.compare_exchange_strong(size_max, m_queue->m_queue->m_head.load())) { if (m_queue->m_queue->m_head_protector.compare_exchange_strong(size_max, m_queue->m_queue->m_head.load())) {
auto old_head = m_queue->m_queue->m_head.load(); auto old_head = m_queue->m_queue->m_head.load();
// This check looks like it's in a weird place (especially since we have to roll back the protector), but it's actually protecting against a race between multiple dequeuers.
if (old_head >= m_queue->m_queue->m_tail.load()) {
m_queue->m_queue->m_head_protector.store(NumericLimits<size_t>::max(), AK::MemoryOrder::memory_order_release);
return QueueStatus::Empty;
}
auto data = move(m_queue->m_queue->m_data[old_head % Size]); auto data = move(m_queue->m_queue->m_data[old_head % Size]);
m_queue->m_queue->m_head.fetch_add(1); m_queue->m_queue->m_head.fetch_add(1);
m_queue->m_queue->m_head_protector.store(NumericLimits<size_t>::max(), AK::MemoryOrder::memory_order_release); m_queue->m_queue->m_head_protector.store(NumericLimits<size_t>::max(), AK::MemoryOrder::memory_order_release);