SCHED_4BSD: maybe_resched() cannot schedule ast() for curthread

[kib]

Summary

SCHED_4BSD: maybe_resched() cannot schedule ast() for curthread

maybe_resched() needs to schedule TDA_SCHED for curthread, but this
requires taking curthread lock while owning some other thread lock.  To
avoid introducing the order
- Use a scheduler-private TDP flag
- register an unconditional TDA_SCHED_PRIV for 4BSD
When an AST needs to be scheduled, i.e. the current thread must do
context switch in the return to userspace path, set the flag.
Then the ast handler calls ast_scheduler(), which gives the same
effect as scheduling TDA_AST.

The overhead is a single function call on each userspace return, for
4BSD case.

Reported and tested by: pho

[kib]

Simplify. Use TDP_SCHED1 instead of the new field in the ts structure.

[markj]

sys/kern/sched_4bsd.c

284

KASSERT that td == curthread here?

[kib]

sys/kern/sched_4bsd.c

284

This is ast handler, so the invariant holds by definition.
But added the MPASS().

[kib]

Add assert that ast handler is called for curthread.

[markj]

sys/kern/sched_4bsd.c

284

the invariant holds by definition

Not quite though, ast_kclear() may call the AST handler. I'm aware it does not do that here, today.