enable irqs when waiting for rwlocks

Hello,

SGI has observed that on large systems, interrupts are not serviced for
a long period of time when waiting for a rwlock. The following patch
series re-enables irqs while waiting for the lock, resembling the code
which is already there for spinlocks.

I only made the ia64 version, because the patch adds some overhead to
the fast path. In particular, I wasn't able to make a reasonably
lightweight implementation for x86, so I assume there are not such large
systems based on x86(_64). Of course, the possibility to implement
raw_{read|write}_lock_flags for any architecture is still there.

Petr Tesarik


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