Add missing thread safety to PagedAllocator

[RandomShaper]

This has involved in the end making SpinLock's functions const, which is also consistent with Mutex.

[lawnjelly]

Wonder if it is possible to have a spinlock guard like MutexLock?

[RandomShaper]

Not a bad idea, but that would be out of the scope of this PR. And, in any case, some of the Godot code is not even using the MutexLock, preferring explicit lock-unlock even when the RAII lock would be usable.

[lawnjelly]

Also, I hate to be that guy 😃 , (and as I know very little about the low level details I feel qualified to make newbie question), should we be using spinlocks now that modern mutexes are very good?

https://matklad.github.io/2020/01/04/mutexes-are-faster-than-spinlocks.html

[RandomShaper]

Also, I hate to be that guy 😃 , (and as I know very little about the low level details I feel qualified to make newbie question), should we be using spinlocks now that modern mutexes are very good?

https://matklad.github.io/2020/01/04/mutexes-are-faster-than-spinlocks.html

This is something I had in my mental (and, I think, in my written) TODO list. I attended a talk in the GDC from AMD people on how to optimize games for Ryzen processors. I think most of what they said is universal. And one of the guidelines (after comparing trivial user spinlock, clever user spinlock, and mutex) was, "Use std::mutex."

So I'd really like that this topic was researched and benchmarked in Godot. If modern mutexes have all the good of both classic OS mutexes and spinlocks, then why not just use them.

[RandomShaper]

In any case, the modification to SpinLock here is incidental. The basic target of this PR is PagedAllocator.

[AThousandShips]

Without getting more off topic than this I'd suggest for researching the spinlock Vs mutex for Godot would be to replace spinlock in cases where they don't necessarily need to be spinlocks (I believe there are some cases where they are still suggested) with a SpinlockOrMutex that can be selected based on platform and be initially conservatively decided (for example I'm not sure how the comparison goes for mobile platforms etc.)

But maybe it's a good idea to open a proposal or discussion to track this

[akien-mga]

Thanks!