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Better fix for Cache race condition

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This commit is contained in:
Sebastian Messmer 2015-10-01 01:52:21 +02:00
parent 90471ea6a3
commit 942a627173
1 changed files with 8 additions and 5 deletions
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13
implementations/caching/cache/Cache.h vendored
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@ -98,7 +98,7 @@ void Cache<Key, Value>::_popOldEntries() {
// to be called in parallel. The call to _popOldEntry() is synchronized to avoid race conditions, // to be called in parallel. The call to _popOldEntry() is synchronized to avoid race conditions,
// but the Value destructor is called in this function which is not synchronized. // but the Value destructor is called in this function which is not synchronized.
// The shared upgrade_lock in here takes care that no push() or pop() operation is running while old entries are deleted. // The shared upgrade_lock in here takes care that no push() or pop() operation is running while an old entry is deleted.
// This would cause race conditions because pop() could return none before the destructor of the deleted element // This would cause race conditions because pop() could return none before the destructor of the deleted element
// has finished running. Since the destructor of a cached newly created block creates it in the base block store, // has finished running. Since the destructor of a cached newly created block creates it in the base block store,
// there would be a state where the block can neither be found by a pop() in the cache, nor in the base store. // there would be a state where the block can neither be found by a pop() in the cache, nor in the base store.
@ -106,10 +106,13 @@ void Cache<Key, Value>::_popOldEntries() {
// The shared lock is then upgraded to a unique lock in _popOldEntry, so that only one thread can work on the // The shared lock is then upgraded to a unique lock in _popOldEntry, so that only one thread can work on the
// _cachedBlocks vector at the same time. // _cachedBlocks vector at the same time.
// There is a regression test case for this: CacheTest_RaceCondition:PopBlocksWhileRequestedElementIsThrownOut. // There is a regression test case for this: CacheTest_RaceCondition:PopBlocksWhileRequestedElementIsThrownOut.
boost::upgrade_lock<boost::shared_mutex> lock(_mutex); while (true) {
boost::optional<Value> oldEntry = _popOldEntry(&lock); boost::upgrade_lock<boost::shared_mutex> lock(_mutex);
while (oldEntry != boost::none) { boost::optional<Value> oldEntry = _popOldEntry(&lock);
oldEntry = _popOldEntry(&lock); if (oldEntry == boost::none) {
break;
}
oldEntry = boost::none; // Call destructor (inside shared lock)
} }
} }