[swift-evolution] [Concurrency] async/await + actors

John McCall rjmccall at apple.com
Tue Sep 12 14:30:32 CDT 2017

> On Sep 12, 2017, at 2:19 AM, Pierre Habouzit via swift-evolution <swift-evolution at swift.org> wrote:
>> On Sep 11, 2017, at 9:00 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> On Sep 4, 2017, at 12:18 PM, Pierre Habouzit <phabouzit at apple.com <mailto:phabouzit at apple.com>> wrote:
>>> Something else I realized, is that this code is fundamentally broken in swift:
>>> actor func foo()
>>> {
>>>     NSLock *lock = NSLock();
>>>     lock.lock();
>>>     let compute = await someCompute(); <--- this will really break `foo` in two pieces of code that can execute on two different physical threads.
>>>     lock.unlock();
>>> }
>>> The reason why it is broken is that mutexes (whether it's NSLock, pthread_mutex, os_unfair_lock) have to be unlocked from the same thread that took it. the await right in the middle here means that we can't guarantee it.
>> Agreed, this is just as broken as:
>> func foo()
>> {
>>     let lock = NSLock()
>>     lock.lock()
>>     someCompute {
>> 	    lock.unlock()
>>     }
>> }
>> and it is just as broken as trying to do the same thing across queues.  Stuff like this, or the use of TLS, is just inherently broken, both with GCD and with any sensible model underlying actors.  Trying to fix this is not worth it IMO, it is better to be clear that they are different things and that (as a programmer) you should *expect* your tasks to run on multiple kernel threads.
>> BTW, why are you using a lock in a single threaded context in the first place??? ;-)
> I don't do locks, I do atomics as a living.
> Joke aside, it's easy to write this bug we should try to have the compiler/analyzer help here for these broken patterns.
> TSD is IMO less of a problem because people using them are aware of their sharp edges. Not so much for locks.

Maybe we could somehow mark a function to cause a warning/error when directly using it from an async function.  You'd want to use that on locks, synchronous I/O, probably some other things.

Trying to hard-enforce it would pretty quickly turn into a big, annoying effects-system problem, where even a program not using async at all would suddenly have to mark a ton of functions as "async-unsafe".  I'm not sure this problem is worth that level of intrusion for most programmers.  But a soft enforcement, maybe an opt-in one like the Clang static analyzer, could do a lot to prod people in the right direction.

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