This is a new futex operation to allow a thread to wait on several
futexes at the same time, and wake up on any of them.  In a sense, it
implements one of the features that was supported by pooling on the old
FUTEX_FD interface.

My use case for this feature lies in Wine, where we want to implement a
similar function available in Windows, mainly for event handling.  The
wine folks have an implementation of the userspace side using eventfd,
but it suffers from bad performance, as shown in the measurements below.

Technically, the old FUTEX_WAIT implementation can be easily
reimplemented using do_futex_wait_multiple, with a count one, and I have
a patch demonstrating how it works.  I'm not proposing it, since futex
is such a tricky code, that I'd be more confortable to have
FUTEX_WAIT_MULTIPLE running upstream for a couple development cycles,
before considering modifying FUTEX_WAIT.

This was tested using three mechanisms:

1) By reimplementing FUTEX_WAIT in terms of FUTEX_WAIT_MULTIPLE and
running tools/testing/selftests/futex and a full linux distro on top of
this kernel.

2) By an example code that exercises the FUTEX_WAIT_MULTIPLE path on a
multi thread, event handling setup.

3) By running the Wine fsync implementation and executing multi-threaded
applications, in particular modern games on top of the implementation.

Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Steven Noonan <steven@valvesoftware.com>
Signed-off-by: Pierre-Loup A. Griffais <pgriffais@valvesoftware.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>