[swift-users] Data races with copy-on-write

Jens Alfke jens at mooseyard.com
Tue Dec 5 13:22:34 CST 2017

> On Dec 5, 2017, at 6:24 AM, Michel Fortin via swift-users <swift-users at swift.org> wrote:
> The array *storage* is copy on write. The array variable (which essentially contains a pointer to the storage) is not copy on write. If you refer to the same array variable from multiple threads, you have a race. Rather, use a different copy of the variable to each thread. Copied variables will share the same storage but will make a copy of the storage when writing to it.

I think you’ve misunderstood. The race condition Romain is referring to is when the two threads both access the shared storage, through separate array variables.

From the thread sanitizer warnings, it sounds like the implementation of Array is not using synchronization around its call(s) to isKnownUniquelyReferenced … which would mean the class is not thread-safe.

It’s pretty common for the regular (mutable) collection classes supplied by a framework to be non-thread-safe; for example consider Foundation and Java. The reason is that the overhead of taking a lock every time you access an array element is pretty high. Generally it’s preferable to use larger-granularity locks, i.e. grab an external lock before performing a number of array operations.

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