bbdaf63d0f
Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/9686 According to https://www.cplusplus.com/reference/deque/deque/back/, " The container is accessed (neither the const nor the non-const versions modify the container). The last element is potentially accessed or modified by the caller. Concurrently accessing or modifying other elements is safe. " Also according to https://www.cplusplus.com/reference/deque/deque/pop_front/, " The container is modified. The first element is modified. Concurrently accessing or modifying other elements is safe (although see iterator validity above). " In RocksDB, we never pop the last element of `DBImpl::alive_log_files_`. We have been exploiting this fact and the above two properties when ensuring correctness when `DBImpl::alive_log_files_` may be accessed concurrently. Specifically, it can be accessed in the write path when db mutex is released. Sometimes, the log_mute_ is held. It can also be accessed in `FindObsoleteFiles()` when db mutex is always held. It can also be accessed during recovery when db mutex is also held. Given the fact that we never pop the last element of alive_log_files_, we currently do not acquire additional locks when accessing it in `WriteToWAL()` as follows ``` alive_log_files_.back().AddSize(log_entry.size()); ``` This is problematic. Check source code of deque.h ``` back() _GLIBCXX_NOEXCEPT { __glibcxx_requires_nonempty(); ... } pop_front() _GLIBCXX_NOEXCEPT { ... if (this->_M_impl._M_start._M_cur != this->_M_impl._M_start._M_last - 1) { ... ++this->_M_impl._M_start._M_cur; } ... } ``` `back()` will actually call `__glibcxx_requires_nonempty()` first. If `__glibcxx_requires_nonempty()` is enabled and not an empty macro, it will call `empty()` ``` bool empty() { return this->_M_impl._M_finish == this->_M_impl._M_start; } ``` You can see that it will access `this->_M_impl._M_start`, racing with `pop_front()`. Therefore, TSAN will actually catch the bug in this case. To be able to use TSAN on our library and unit tests, we should always coordinate concurrent accesses to STL containers properly. We need to pass information about db mutex and log mutex into `WriteToWAL()`, otherwise it's impossible to know which mutex to acquire inside the function. To fix this, we can catch the tail of `alive_log_files_` by reference, so that we do not have to call `back()` in `WriteToWAL()`. Reviewed By: pdillinger Differential Revision: D34780309 fbshipit-source-id: 1def9821f0c437f2736c6a26445d75890377889b |
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RocksDB: A Persistent Key-Value Store for Flash and RAM Storage
RocksDB is developed and maintained by Facebook Database Engineering Team. It is built on earlier work on LevelDB by Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com)
This code is a library that forms the core building block for a fast key-value server, especially suited for storing data on flash drives. It has a Log-Structured-Merge-Database (LSM) design with flexible tradeoffs between Write-Amplification-Factor (WAF), Read-Amplification-Factor (RAF) and Space-Amplification-Factor (SAF). It has multi-threaded compactions, making it especially suitable for storing multiple terabytes of data in a single database.
Start with example usage here: https://github.com/facebook/rocksdb/tree/main/examples
See the github wiki for more explanation.
The public interface is in include/. Callers should not include or
rely on the details of any other header files in this package. Those
internal APIs may be changed without warning.
Questions and discussions are welcome on the RocksDB Developers Public Facebook group and email list on Google Groups.
License
RocksDB is dual-licensed under both the GPLv2 (found in the COPYING file in the root directory) and Apache 2.0 License (found in the LICENSE.Apache file in the root directory). You may select, at your option, one of the above-listed licenses.