[DRAFT] Add ReadWrite locking mechanism on top of Blockchain. #9173
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jeffro256
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Feb 15, 2024
src/cryptonote_core/blockchain.cpp
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| m_blockchain_transaction.start_write(); | ||
| epee::misc_utils::auto_scope_leave_caller scope_exit_handler = | ||
| epee::misc_utils::create_scope_leave_handler([&](){ | ||
| m_blockchain_transaction.end_write(); | ||
| }); |
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I feel like this pattern is a good candidate for an RAII class. This class could also let you lock and unlock more finely within this scope depending on the operations required.
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I disagree here. Eventually, we want to have a fine-grained locking mechanism. RAII here would just make this a lot more cumbersome to use.
Consider this as prototype/experimental PR, which does have bugs, that is why I am using auto_scope_leave_caller here. Eventually, we want to only lock where we interact with write/reads. Not whole functions.
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You can still have fine-grained locking with RAII. Consider std::unique_lock.
src/cryptonote_core/blockchain.cpp
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| @@ -1102,7 +1136,11 @@ size_t Blockchain::recalculate_difficulties(boost::optional<uint64_t> start_heig | |||
| //------------------------------------------------------------------ | |||
| std::vector<time_t> Blockchain::get_last_block_timestamps(unsigned int blocks) const | |||
| { | |||
| CRITICAL_REGION_LOCAL(m_blockchain_lock); | |||
| m_blockchain_transaction.start_write(); | |||
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Why are all these start_write() even for functions which only read data?
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Consider this as prototype/experimental PR, which does have bugs.
This is a symptom of a bigger problem we have. Imagine this scenario: Functions calls: A->B->C->D.
A is a write transaction. But B, C and D are read transactions. We have to acquire a write lock for B, C, and D since read and write transactions in this implementation are exclusive. (I am fixing it).
Piggybacking on the previous comment, if we do implement more fine-grained locking we would not have this problem.
src/cryptonote_core/blockchain.h
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| boost::condition_variable_any r_condition; | ||
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| boost::recursive_mutex w_mutex; | ||
| std::atomic<unsigned int> w_locks; |
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Why would there allowed to be more than 1 write lock at a time?
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Recursive function calls.
E.g. Imagine this function calls: A->B->C->D.
A, B, and C are a write transaction. But D is read transactions.
(this will be fixed too).
One other approach would just check boost::thread::id and if you are the owner then don't lock again. That's one approach. I am evaluating which way we should go.
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How much would the code have to be refactored so that we don't use the locks recursively in the first place? (probably a lot)
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That is one of the cleaner approaches and end result of that approach would be much much cleaner. But the problem with that is a lot of APIs should change.
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take a look at this line:
This is much cleaner. But at the moment, it is not possible since we have to change a lot of APIs.
Eventually, we want that. A transaction-based blockchain.cpp. Transaction objects will be trivial to create and you can create ReadTransaction and WriteTransaction. For now, this is an intermediary patch to improve.
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@jeffro256 thanks for your comment. I left some comments too. |
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Leaving this here just to keep in mind. This deadlock should be fixed: Thread 1, waiting while holding the lock: Thread 2, locking while other thread has the lock: |
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This will fix existing locking issues, and provide a foundation to implement more fine-grained locking mechanisms in future works.
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This will fix existing locking issues, and provide a foundation to implement more fine-grained locking mechanisms in future works.