umtx: Check for errors from suword32()

[markj]

Summary

This is in preparation for annotating copyin() and related functions
with __result_use_check.

[kib]

sys/kern/kern_umtx.c

2970

I propose to write the last three line as error = EFAULT; goto out' with the obvious place for the out label. The function has single return place.

3222

Now this really asks for the CAS loop. Might be it would be a useful primitive in kern_umtx.c the wrapper around casueword32 which handles thread_check_susp().

[markj]

sys/kern/kern_umtx.c

3222

I don't quite follow - why do we want CAS here? This is just an atomic decrement.

[markj]

Handle kib's note.

[kib]

sys/kern/kern_umtx.c

3222

There is no atomic decrement op for userspace, We have to implement all of them as CAS.