[swift-dev] Thread safety of weak properties

Mike Ash mike at mikeash.com
Mon Dec 14 21:51:55 CST 2015

> On Dec 14, 2015, at 3:19 PM, Greg Parker via swift-dev <swift-dev at swift.org> 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.

Do you have a pointer (unintentional pun, oops) to this algorithm?

In this case, if we're going to dedicate a whole word to it, then we might as well go with the activity count implementation, but I'd be curious to read more just the same.


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