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Diffstat (limited to 'source/blender/blenlib/BLI_cache_mutex.hh')
-rw-r--r-- | source/blender/blenlib/BLI_cache_mutex.hh | 106 |
1 files changed, 106 insertions, 0 deletions
diff --git a/source/blender/blenlib/BLI_cache_mutex.hh b/source/blender/blenlib/BLI_cache_mutex.hh new file mode 100644 index 00000000000..8e2a0d1b1a5 --- /dev/null +++ b/source/blender/blenlib/BLI_cache_mutex.hh @@ -0,0 +1,106 @@ +/* SPDX-License-Identifier: GPL-2.0-or-later */ + +#pragma once + +/** + * A #CacheMutex is used to protect a lazily computed cache from being computed more than once. + * Using #CacheMutex instead of a "raw mutex" to protect a cache has some benefits: + * - Avoid common pitfalls like forgetting to use task isolation or a double checked lock. + * - Cleaner and less redundant code because the same locking patterns don't have to be repeated + * everywhere. + * - One can benefit from potential future improvements to #CacheMutex of which there are a few + * mentioned below. + * + * The data protected by #CacheMutex is not part of #CacheMutex. Instead, the #CacheMutex and its + * protected data should generally be placed next to each other. + * + * Each #CacheMutex protects exactly one cache, so multiple cache mutexes have to be used when a + * class has multiple caches. That is contrary to a "custom" solution using `std::mutex` where one + * mutex could protect multiple caches at the cost of higher lock contention. + * + * To make sure the cache is up to date, call `CacheMutex::ensure` and pass in the function that + * computes the cache. + * + * To tell the #CacheMutex that the cache is invalidated and to be re-evaluated upon next access + * use `CacheMutex::tag_dirty`. + * + * This example shows how one could implement a lazily computed average vertex position in an + * imaginary `Mesh` data structure: + * + * \code{.cpp} + * class Mesh { + * private: + * mutable CacheMutex average_position_cache_mutex_; + * mutable float3 average_position_cache_; + * + * public: + * const float3 &average_position() const + * { + * average_position_cache_mutex_.ensure([&]() { + * average_position_cache_ = actually_compute_average_position(); + * }); + * return average_position_cache_; + * } + * + * void tag_positions_changed() + * { + * average_position_cache_mutex_.tag_dirty(); + * } + * }; + * \endcode + * + * Possible future improvements: + * - Avoid task isolation when we know that the cache computation does not use threading. + * - Try to use a smaller mutex. The mutex does not have to be fair for this use case. + * - Try to join the cache computation instead of blocking if another thread is computing the cache + * already. + */ + +#include <atomic> +#include <mutex> + +#include "BLI_function_ref.hh" + +namespace blender { + +class CacheMutex { + private: + std::mutex mutex_; + std::atomic<bool> cache_valid_ = false; + + public: + /** + * Make sure the cache exists and is up to date. This calls `compute_cache` once to update the + * cache (which is stored outside of this class) if it is dirty, otherwise it does nothing. + * + * This function is thread-safe under the assumption that the same parameters are passed from + * every thread. + */ + void ensure(FunctionRef<void()> compute_cache); + + /** + * Reset the cache. The next time #ensure is called, it will recompute that code. + */ + void tag_dirty() + { + cache_valid_.store(false); + } + + /** + * Return true if the cache currently does not exist or has been invalidated. + */ + bool is_dirty() const + { + return !this->is_cached(); + } + + /** + * Return true if the cache exists and is valid. + */ + bool is_cached() const + { + return cache_valid_.load(std::memory_order_relaxed); + } +}; + +} // namespace blender |