rwlocks: poor performance with adaptive spinning

Recently several people have reported problems of starvation with rwlocks.
In particular, users which tried to use rwlock on big SMP environment
(16+ CPUs) found them rather subjected to poor performances and to
starvation of waiters.

Inspecting the code, something strange about adaptive spinning popped
up: basically, for rwlocks, adaptive spinning stubs seem to be
customed too down in the decisioning-loop.
The desposition of the stub will let the thread that would adaptively
spin, to set the respecitve (both read or write) waiters flag on,
which means that the owner of the lock will go down in the hard path
of locking functions and will performe a full wakeup even if the
waiters queues can result empty. This is a big penalty for adaptive
spinning which can make it completely useless.
In addiction to this, adaptive spinning only runs in the turnstile
spinlock path which is not ideal.
This patch ports the approach alredy used for adaptive spinning in sx
locks to rwlocks:
http://users.gufi.org/~rookie/works/patches/kern_rwlock.diff

In sx it is unlikely to see big benefits because they are held for too
long times, but for rwlocks situation is rather different.
I would like to see if people can do benchmarks with this patch (maybe
in private environments?) as I'm not able to do them in short times.

Adaptive spinning in rwlocks can be improved further with other tricks
(like adding a backoff counter, for example, or trying to spin with
the lock held in read mode too), but we first should be sure to start
with a solid base.

Thanks,
Attilio

-- 
Peace can only be achieved by understanding - A. Einstein
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