kdb: Handle process enumeration before procinit()

[mhorne]

Summary

Make kdb_thr_first() and kdb_thr_next() return sane values if the
allproc list and pidhashtbl haven't been initialized yet. This can
happen if the debugger is entered very early on, for example with the
'-d' boot flag.

This allows remote gdb to attach at such a time, and fixes some ddb
commands like 'show threads'.

[imp]

This seems sane to me.

[markj]

sys/kern/subr_kdb.c

612

Why not check pidhashtbl != NULL instead?

656

Similarly here, we could check pidhashtbl != NULL after checking TAILQ_NEXT(thr, td_plist).

[mhorne]

sys/kern/subr_kdb.c

612

Sure, this is slightly preferable.

656

Why is it better after this check? It doesn't seem possible for td_plist to be populated but pidhashtbl be NULL.

[markj]

sys/kern/subr_kdb.c

656

It's not possible today, it just seemed very slightly cleaner to me to not make that assumption, supposing that someone might someday add a static thread1 to proc0 or something.

[mhorne]

Update with markj's feedback. Keep the allproc initialization bit, as it may be checked early by db_ps().

[mhorne]

Also give pidhashtbl a specific initial value of NULL. NFC, but semantically correct.