thread pool helpers

[tridge at samba.org]

Hi Volker,

 > Functionally this should not hurt us then, because in the
 > first run we would only do the fd-based calls in helper
 > threads.

I think it still can hurt, as it means that while one of these fd
based calls is outstanding, any setreuid() and similar calls made by
smbd to process other requests will go very slow.

The good news is that unlike the signal base AIO code, this at least
it wouldn't actually break things (or at least as far as I can see).

 > And with a clone-based implementation we could even do the
 > path-based calls in helper threads, with a helper pool per session
 > setup?

yep

 > Remains the question of the memory barriers :-)

I guess we'd need to learn to use the low level api to futexes. That
might lead to some fairly subtle code :-)

Cheers, Tridge