Date: Thu, 15 Dec 2005 08:17:42 -0800 From: rookie <asmrookie@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-smp@freebsd.org Subject: Re: Use turnstile to implement sx_lock Message-ID: <3bbf2fe10512150817s346d621do@mail.gmail.com> In-Reply-To: <200512151017.12168.jhb@freebsd.org> References: <1fa17f810512150652h5da6a6a5g3347f841a614689e@mail.gmail.com> <200512151017.12168.jhb@freebsd.org>
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2005/12/15, John Baldwin <jhb@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 rwlo= ck > 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 -- Peace can only be achieved by understanding - A. Einstein
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