When the world was a simple and static place setting up irqs was easy.
It sufficed to allocate a linux irq number and a find a free cpu
vector we could receive that linux irq on.  In those days it was
a safe assumption that any allocated vector was actually in use
so after one global pass through all of the vectors we would have
none left.

These days things are much more dynamic with interrupt controllers
(in the form of MSI or MSI-X) appearing on plug in cards and linux
irqs appearing and disappearing.  As these irqs come and go vectors
are allocated and freed,  invalidating the ancient assumption that all
allocated vectors stayed in use forever.

So this patch modifies the vector allocator to walk through every
possible vector before giving up, and to check to see if a vector
is in use before assigning it.  With these changes we stop leaking
freed vectors and it becomes possible to allocate and free irq vectors
all day long.

This changed was modeled after the vector allocator on x86_64 where
this limitation has already been removed.  In essence we don't update
the static variables that hold the position of the last vector we
allocated until have successfully allocated another vector.  This
allows us to detect if we have completed one complete scan through
all of the possible vectors.
Acked-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>