Re: [RFC] Userspace RCU: (ab)using futexes to save cpu cycles and energy

[Michael Schnell <mschnell@lumino.de>]

Mathieu Desnoyers wrote:
> Hrm, your assumption about the common case does not seem to fit my
> scenarios. 
Yep,

Obviously I was misled.

So I now understand that you want to schedule the thread as a result to
some event and the Thread might already be running at that time, so that
it does not need to enter an OS-based wait state.

But to me this looks as if a _counting_ semaphore is needed here (at
least in a more general case), instead of a binary semaphore (such as
FUTEX does manage). Only with a counting semaphore no events are missed
(unless the user software design cares for this with other means, which
might be difficult or impossible).

Of course the fast path of a  user space counting semaphore is easily
doable with atomic_inc() and atomic_dec().

But I only know about two working variants of the user space code for
the (binary) FUTEX. There seem to be some more implementations that do
not work decently.

I have no idea if it's possible to create a counting semaphore in user
space that uses the "futex" syscall (or whatever) for the case the
threads needs to wait.

-Michael