[swift-evolution] [Proposal] Threadsafe lazy vars

Andrew Trick atrick at apple.com
Wed Apr 6 18:34:17 CDT 2016

> On Apr 6, 2016, at 2:07 PM, Michael Peternell via swift-evolution <swift-evolution at swift.org> wrote:
> Hi all,
> lazy vars are not threadsafe in Swift 2. I saw code that uses lazy initialization in an unsafe way that introduces race conditions so often that I would be rich by now if get a penny each time. Many people use patterns like { if(_var == nil) { _var = [self _calculateVar]; } return _var; } or they just dispatch_once, but forget that they are in an instance method, and that it will all break if there is ever more than one instance of that class.
> I propose to make lazy vars atomic. Optionally, the old lazy var behavior could be renamed to something like lazy_nonatomic.
> I want to list some pros and cons for making lazy vars threadsafe:
> Pros:
> - This proposal will not change the behavior of programs which are free from data races. I could argue that the change is therefore backwards-compatible.
> - I would say that programs which require lazy vars to be nonatomic in order to function correctly, are really bad style; threadsafe lazy vars behave much more deterministic. Many programs which use lazy vars incorrectly could suddenly become safe if this proposal is implemented.
> - The overhead would be minimal. For example, suppose we have a lazy var of type `NSImage`. We could represent that variable as a simple pointer which is initialized to NULL. The access could look something like this (this is just an example, there may be even more efficient solutions): {
>    // we need to make sure that reads on _var are not cached:
>    memory_read_barrier(&_var);
>    // ^^and I'm not 100% sure that we really need that memory barrier.
>    // (at least it's not needed for static vars, as proven by the implementation of dispatch_once())
>    if(_var == nil) {
>        @synchronized(&_var) {
>            // ^^we synchronize on &_var, and not on _var
>            // this is semantically invalid in objc, but the objc-runtime supports it.
>            // The point I want to make is that we don't need extra storage for the
>            // synchronization, in many cases.
>            if(_var != nil) {
>                return _var;
>            }
>            ... some code that initializes _var
>        }
>        //@synchronized() already employs memory barriers, so no additional barriers are needed
>        //maybe we should use a non-recursive lock though..
>    }
>    return _var;
> }
> - Currently, if you need threadsafety, you cannot use lazy. You can of course wrap a lock around a nonatomic lazy var, but that would be much more inefficient than a native implementation.
> - I guess, no one will really complain if lazy var's are suddenly threadsafe. I also cannot see how it would break any code (except for contrived examples.)
> - In some cases, the nonatomic behavior can be used as an optimization, if it is semantically equivalent. For example, a lazy var that lives in automatic storage (i.e. not an ivar or static var, but just a local var) and that is *not* captured in a closure expression can be safely initialized in a non-threadsafe way, because the variable can not be accessed from more than one thread concurrently anyways.
> Cons:
> - This would be the first concurrency primitive built into the language (at least as far as I know)
> - It may suggest to users of the language that other primitives (like var's) would be threadsafe too, which is obviously not the case.
> - There is at least *some* runtime overhead involved. It's not zero-cost. On the other hand, lazy initialization should only be used when the cost of initialization is much higher than the cost of creating and maintaining a thunk. And in that case, I think the performance characteristics are pretty well.
> - It may be out of scope for Swift 3 :-(
> Proposed solution:
>    public lazy var foo: Type = fn()
> is semantically equivalent to
>    private var _lazy_storage_foo: Type?
>    private var _lazy_lock_foo: Lock
>    public var foo: Type {
>        get {
>            var result: Type?
>            _lazy_lock_foo.withLock {
>                if(_lazy_storage_foo == nil) {
>                    _lazy_storage_foo = fn()
>                }
>            }
>            return _lazy_storage_foo!
>        }
>    }
> except that the builtin solution is much more efficient, and that the two private extra vars are not exposed when you use the lazy keyword.
> All in all, I think that threadsafe lazy vars would be a nice feature for the language. I welcome feedback and am interested in a discussion.

These are good points. I think we need both nonatomic and atomic lazy variables. The syntax and scaffolding will likely fall out of Property Behaviors:
https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md <https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md>

All that’s left would be optimizing the implementation, which would be premature to discuss.

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