Use turnstile to implement sx_lock

[rookie]

2005/12/15, John Baldwin <jhb at freebsd.org>:
>
> You have to add a second queue to the turnstile and make priority
> propagation
> still work, etc.  Mutexes would just use the exclusive queue all the time
> whereas rwlocks would use both queues.  This is the hard part of the rwlock
> project.  I've sort-of started on this but haven't gotten very far at all in
> my jhb_lock p4 branch.

I'm working on the same problem too and I found another solution
beacause, in order to mantain a clean design, maybe modifying
turnstiles code is not the better idea.
If we have a thread trying to slock it just blocks if sx is held in
"exclusive mode" by another thread so it's enough a simple turnstile
as for other blocking locks and we have nice priorty propagation. The
real problem is when we try to xlock. A xlocking thread blocks if sx
is held in "shared mode" (even by different owners) so we might
mantain a track of every slocking thread (through a tailqueue, a
static array or whatever) in order to have a priority propagation for
those. I think (I didn't implement yet) it can be done outside the
turnstile context so it seems we don't necessary need to modify.
Finally, we might consider one thing: turnstile has just one owner
while we have (in the slocks) more than one and so what's the real
owner?
We could use a "head thread" per track which must be treacted
carefully (EG: on slocking it we might switch the turnstile owner).

What do you think about?

Attilio

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