什么时候可以安全地访问互斥锁保护的变量而无需锁定？

Question

A common pattern of storing config in my code is a "map[string]interface{}" protected by RWMutex, but usually after app initiated (could be triggered in multiple go-routine), the map becomes totally readonly. So I have a feeling that from some point of time on, the RWMutex on read should be unnecessary.

An example of this config map is at http://play.golang.org/p/tkbj9DBok_

One fact that brought me to think of this is in some of production code it actually doing this way of unprotected access of shared object (though it's mostly readonly after it's init'ed), I understand normal way of using RWMutex to protect, but interesting part is this malformed code haven't run into problem in past months.

Is that true that after some accurate "time point" that writes are flushed from cache into memory and with a guarantee of no more writes needed, reads can actually go without RWMutex.RLock? If YES, when is the time point or how to setup the conditions before lockless access?

Answer 1

As long as no one is modifying the map, it should be safe for multiple threads to read it at once. Unfortunately, without any locking you'll have no way to make sure no one else is reading the map when you want to update it.

So one solution is to never update the map, but instead replace it atomically. The read-copy-update algorithm could be used here. Rather than directly accessing the map, so you need to dereference the pointer to access the map. To update it, you can do the following:

1. acquire an "update lock" mutex.
2. make a copy of the map. You want to copy all keys/values manually: simple assignment won't work because maps are reference types.
3. make your changes to the copy of the map.
4. use StorePointer from the sync/atomic package to atomically update the pointer to the live map to point to your new map.
5. release the mutex.

Everything that runs before the atomic update in (4) will see the old map, and everything after will see the new map. At no point will those goroutines be reading from a map that is being written to, so there is no need for an RWMutex.