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The Pillars of Concurrency


Pillar 3: Consistency Via Safely Shared Resources

Pillar 3 is all about dealing with shared resources, especially shared memory state, without either corruption or deadlock. Here we use key terms like acquire and release; read and write; and atomic and consistent and transaction. In these columns, I'll mostly focus on dealing with mutable objects in shared memory.

Today's status quo for synchronizing access to mutable shared objects is locks. Locks are known to be inadequate (see [3] and [4]), but they are nevertheless the best general-purpose tools we have. Some frameworks provide selected lock-free data structures (hash tables) that are internally synchronized using atomic variables so that they can be used safely without taking locks either internally inside the data structure implementation or externally in your calling code; these are useful, but they are not a way to avoid locking in general because they are few and many common data structures have no known lock-free implementations at all.

In the future, we can look forward to improved support for locks (for example, being able to express lock levels/hierarchies in a portable way, and what data is protected by what lock) and probably transactional memory (where the idea is to automatically version memory, so that the programmer can just write "begin transaction; do work; end transaction" like we do with databases and let the system handle synchronization and contention automatically). Until we have those, though, learn to love locks.


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