mtx_lock_recurse/mtx_unlock_recurse functions (proof-of-concept).

[Pawel Jakub Dawidek]

Hi.

As was discussed, it will be helpful to have functions, that are able
to acquire lock recursively, even if lock itself isn't recursable.

Here is a patch, that should implement this functionality:

	http://people.freebsd.org/~pjd/patches/mtx_lock_recurse.patch

I also added a KASSERT() to protect against mixed locking modes, e.g.:

	mtx_lock(&mtx);
	[...]
	mtx_lock_recurse(&mtx);
	[...]
	mtx_unlock(&mtx);
	[...]
	mtx_unlock_recurse(&mtx);

This is not permitted, but this should work just fine:

	mtx_lock(&mtx);
	[...]
	mtx_lock_recurse(&mtx);
	[...]
	mtx_unlock_recurse(&mtx);
	[...]
	mtx_unlock(&mtx);

-- 
Pawel Jakub Dawidek                       http://www.FreeBSD.org
pjd at FreeBSD.org                           http://garage.freebsd.pl
FreeBSD committer                         Am I Evil? Yes, I Am!
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