[OpenAFS-devel] Help, please: rx_EnableHotThread() forgets thread IDs in pthreaded

[Nickolai Zeldovich]

> Curiously, this is a one-way change, not a swap, with two threads ending up 
> with the same thread-id afterwards. I fail to see a replenishing mechanism 
> which would prevent the system from evolving to a single-thread-id 
> constellation over time.

At least in theory, I think I understand how it's supposed to work..
This swap only happens between the listener thread T1 and a worker
thread T2, whereby the listener thread takes over the worker thread's
thread-id.  The listener thread doesn't have a unique thread-id of
its own, since it doesn't need one -- it always stays in Rx code.
After the swap, the listener thread T1 becomes a worker thread,
and worker thread T2 becomes a listener thread.  When the new listener
thread T2 decides to become a worker thread (via the same thread-ID
switch), it will acquire another worker thread's thread-ID, and the
other worker thread will become the listener.

Now, it seems like there's a bug here somewhere..  Maybe we should
solve the problem by giving the listener thread a unique thread-ID
of its own, and then we would never need to swap thread-IDs when
doing hot-thread switching.

> Of course the next step is to change rx.c and SWAP the tno's - but I'm afraid 
> that this code is actually executed pretty rarely and a swap might just
> create a problem that passes unnoticed for a long time.

I suspect in the case of a pthread fileserver with hotthread enabled,
this code gets executed almost all the time..

-- kolya