5 ms·
That's why I opened with "Look at the implementation". Go is unable to store the type and the pointer at the same time, so it warps what "atomic" means. Pret
by bumper_crop 4y ago
That's why I opened with "Look at the implementation". Go is unable to store the type and the pointer at the same time, so it warps what "atomic" means. Pretty much every other language has atomic mean "one of these will win, one will lose". Go says "one will win, one will panic and destroy the goroutine.
In fact, it's even worse than that. If the Store() caller goes to sleep between setting the type and storing the pointer, it causes every Goroutine that calls Load() to block. They can't make forward progress if the store caller hangs.
- fastest963 4y agoThis is why all the examples call Store immediately with a zero value of the type.
- bumper_crop 4y agohttps://go.dev/play/p/xolc9oPwA0C https://go.dev/play/p/xolc9oPwA0C Interfaces don't have a zero type, which means that we can't have an atomic.Value which stores Shape. Atomic Value would be much easier to reason about if it had store semantics similar to a regular `var foo Shape = ...`. One of the other comment threads talked about generics helping this, so maybe there is hope.
- deleted 4y ago[deleted]
- yencabulator 4y agoParent means var bestShape atomic.Value bestShape.Store((*Circle)(nil))
- morelisp 4y agoWhich will store it as a *Circle, and only allow more *Circles, not Shapes. That part of GP’s claim is correct. It just had nothing to do with atomicity; it means something specific, not just “I like the failure mode.”
- yencabulator 4y agoThat's pretty easy to workaround: type shapeContainer struct { Shape } The usual way to use atomic.Value is by writing strongly-typed wrappers anyway, so that doesn't affect your codebase beyond about 3 lines.
- deleted 4y ago[deleted]
- morelisp 4y agoAbend is a fairly normal and in many ways best way to "lose" in a race. It's fine, it's atomic.
- deleted 4y ago[deleted]
- tapirl 4y ago> Pretty much every other language has atomic mean "one of these will win, one will lose" Could you elaborate how "much every other language" implement it?
- deleted 4y ago[deleted]
- yencabulator 4y ago> If the Store() caller goes to sleep between setting the type and storing the pointer, it causes every Goroutine that calls Load() to block. Where does this go to sleep: https://cs.opensource.google/go/go/+/refs/tags/go1.18.3:src/sync/atomic/value.go;l=47-83 https://cs.opensource.google/go/go/+/refs/tags/go1.18.3:src/... It looks like a CAS busy loop with preemption disabled, to me.
- bumper_crop 4y agoMake sure to read between the lines. It only looks like a busy loop. Remember, the OS can pause and preempt your thread at any time. This is a real and likely event.
- mbnull 4y agoBy reading the lines and not between them, you could read these two lines: runtime_procPin() and runtime_procUnpin(). With explicit comments that these pause preemption.