Synchronization is Hard

I read a newspaper story about a neuropsychologist who had a stroke. She recounted trying to call 911, but not being able to figure out which digit was which on the phone, or what the steps were to make a phone call. She knew what the right tool was, but she'd lost the cognitive tools to use it. All the while, being a neuropsychologist, she was aware of what was going on, and even somewhat fascinated by it, but also aware that her life depended on doing a seemingly simple thing that she nonetheless could not quite grok.

Synchronization is like this for me. At least I know I'm not alone - some of the smartest people I know have a hard time thinking about synchronization problems, and my industry is littered with bugs due to incorrect synchronization. But I always feel like there's a right way to reason about these problems, and I know it's there but I don't know what it is and I can't even quite articulate why it is that I can't think clearly about it. My hope is that one day I'll GET IT and then I won't be able to remember why I couldn't figure it out before.

I do know some wrong ways to think about synchronization, though. Any time I am reasoning about synchronization and I find myself thinking "okay, if two threads come in here at the same time...", I am about to make a mistake or go down a rat hole. This is how books always present the topic, but intuitively I think it's wrong - I don't believe you can think correctly about synchronization by thinking about execution.

Instead, I think it's probably better to reason in terms of state. "What states could this object be in when this variable is evaluated?" "If I modify the state, how will other threads discover the modification?"

Today I spent a couple hours trying, along with some people who are pretty good at these things, to come up with a good pattern for lazy initialization when the initialization routine is not trusted (e.g., when it might try to call back into the object being initialized). The real moral of the experience is twofold: first, we each wrote routines that we thought were good and that were promptly found wrong by the others; second, although I think we did end up with two valid solutions, I'm not sure how to PROVE that they're valid.