Double-Checked Locking and Objective-C

A common problem in software is to do something exactly once – e.g. create a singleton.  A corollary of that action is to do something exactly once in the life of an instance.  I have found myself using the corollary much more frequently as I move away from using singletons, which I’ve done to improve testability.

With singletons, a common solution is to use locking or, to avoid the performance penalty of locking, double-checked locking.  Of course, double-checked locking has issues with most modern languages caused by CPU and compiler instruction reordering.  Objective-C has provided an elegant solution to those problems, but the solution comes up a little bit short.

Creating a Singleton – Single-Threaded

In a single -threaded application, creating a singleton can be done without additional protection, like this:

+ (MyClass *) sharedInstance
{
    static MyClass * s_sharedInstance = nil;
    if (s_sharedInstance == nil)
    {
        s_sharedInstance = [MyClass new];
    }

    return s_sharedInstance;
}

Creating a Singleton – Multi-Threaded

In a multi-threaded application, it’s not so simple as you will want to protect yourself against multiple callers of this method.  A simple solution simply adds a lock:

+ (MyClass *) sharedInstance
{
    static MyClass * s_sharedInstance = nil;
    @synchronized(self)
    {
        if (s_sharedInstance == nil)
        {
            s_sharedInstance = [MyClass new];
        }
    }

    return s_sharedInstance;
}

The problem with this solution is that you pay the price of locking even though you only need it the first time the method is called.

Double-Checked Locking

A common solution to avoid the penalty of locking is to use double-checked locking.  With double-checked locking you check the value of the static instance both outside and inside the lock, like this:

+ (MyClass *) sharedInstance
{
    static MyClass * s_sharedInstance = nil;
    if (s_sharedInstance == nil)
    {
        @synchronized(self)
        {
            if (s_sharedInstance == nil)
            {
                s_sharedInstance = [MyClass new];
            }
        }
    }

    return s_sharedInstance;
}

At first blush, it would seem that this would solve the problem of assigning the static variable AND avoiding the penalty of the lock.  The problem is that the CPU and/or the compiler could reorder the memory access instructions.  A solution suggested by Mike Ash inserts memory barriers before accessing the variable and uses the volatile keyword.  The memory barrier, available in libkern/OSAtomic.h, solves the CPU problem and the volatile keyword solves the compiler problem.

+ (MyClass *) sharedInstance
{
    static MyClass * volatile s_sharedInstance = nil;
    if (s_sharedInstance == nil)
    {
        @synchronized(self)
        {
            if (s_sharedInstance == nil)
            {
                OSMemoryBarrier();
                s_sharedInstance = [MyClass new];
            }
        }
    }

    OSMemoryBarrier();
    return s_sharedInstance;
}

The problem with this solution is that it is still quite expensive in the most common case.  Apple has provided another solution:  dispatch_once.

dispatch_once

dispatch_once is provided with Grand Central Dispatch to execute statements exactly once, like this:

+ (MyClass *) sharedInstance
{
    static MyClass * s_sharedInstance = nil;

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        s_sharedInstance = [MyClass new];
    });

    return s_sharedInstance
}

This is a terrific solution for use with singletons, but what if you want to use it with an instance?  It actually happens to work, but Apple has a warning against using it in this way in the documentation for dispatch_once:

The predicate must point to a variable stored in global or static scope. The result of using a predicate with automatic or dynamic storage (including Objective-C instance variables) is undefined.

Hmm, that’s unfortunate.  Here’s hoping Apple will change their implementation.  In the meantime, it would appear that the only solutions for the non-static and non-global case are:

  1. Use a synchronized block for every call.
  2. Use memory barriers.

References

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