[swift-dev] Thread safety of weak properties

[Kevin Ballard]

On Mon, Dec 14, 2015, at 07:34 PM, Greg Parker wrote:
> 
> > On Dec 14, 2015, at 7:26 PM, Kevin Ballard via swift-dev <swift-dev at swift.org> wrote:
> > 
> >> On Mon, Dec 14, 2015, at 12:19 PM, Greg Parker via swift-dev wrote:
> >> 
> >>> On Dec 14, 2015, at 9:47 AM, John McCall via swift-dev <swift-dev at swift.org> wrote:
> >>> 
> >>>> On Dec 12, 2015, at 7:04 PM, Chris Lattner <clattner at apple.com> wrote:
> >>>> #3 sounds like a great approach to me.  I agree with Kevin that if we keep the object husk approach that any use of a weak pointer that returns nil should drop any reference to a husk.
> >>> 
> >>> Spin locks are, unfortunately, illegal on iOS, which does not guarantee progress in the face of priority inversion.
> >> 
> >> There is a spinlock algorithm that does work (in practice if not in theory), but it requires a full word of storage instead of a single bit.
> > 
> > Is that what OSSpinLock uses?
> 
> It does not. OSSpinLock is unsafe unless you can guarantee that all users have the same priority.

Hmm, that's pretty unfortunate to hear. I've written code with spinlocks on iOS, and I imagine I'm not the only one. Does the system provide an implementation of this "safe in practice" spinlock that's visible to third-party devs?

-Kevin Ballard