Use turnstile to implement sx_lock

[rookie]

> After some thinking this strikes me as wrong.  You must not grant a slock
> attempt if there is a thread with a higher priority waiting to get a xlock.
> This does not mean that your approach doesn't work, but you have to keep
> this
> in mind.
Yes, it's right.

[snip]
>
> It seems to me that you are talking about the same things with slightly
> different implementation details.  It doesn't really matter much where the
> queue of slocking threads is kept as long as it is easily and efficient to
> update the turnstile's owner on sunlock.  In practice it should be easier to
> have it in the turnstile as well and I still don't really understand why you
> (rookie) do not want to change turnstiles.
In order to mantain current code for mutex (less changes mean less problems).
However I think that a good start point would be writing code for a
new primitive (as John and Max suggested) and I will concentrate my
work there.

Attilio


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