Temporarily pinning a thread to a capability

[Edward Kmett]

Would anything go wrong with a thread id if I pinned it to a capability
after the fact?

I could in theory do so just by setting

tso->flags |= TSO_LOCKED

and then disabling this later by restoring the TSO flags.

I can't think of anything but I figured folks here might be able to think
of invariants I don't know about.

Usage scenario:

I have a number of things where I can't afford a map from a ThreadId# or
even its internal id to a per-thread value for bounded wait-free
structures.

On the other hand, I can afford one entry per capability and to make a
handful of primitives that can't be preempted, letting me use normal
writes, not even a CAS, to update the capability-local variable in a
primitive (indexing into an array of size based on the number of
capabilities). This lets me bound the amount of "helpers" to consider by
the capability count rather than the potentially much larger and much more
variable number of live threads.

However, I may need to access this stuff in "pure" code that wasn't written
with my needs in mind, so I need to at least temporarily pin the current
thread to a fixed capability for the duration when that happens.

This isn't perfect, it won't react to a growing number of capabilities
nicely in the future, but it does handle a lot of things I can't do now at
all without downgrading to lock-free and starving a lot of computations, so
I'm hoping the answer is "it all works". =)

-Edward
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