Date: Fri, 2 May 1997 08:19:20 -0400 (EDT) From: Peter Dufault <dufault@hda.com> To: phk@dk.tfs.com (Poul-Henning Kamp) Cc: current@freebsd.org Subject: Re: vnode->v_usage Message-ID: <199705021219.IAA27851@hda.hda.com> In-Reply-To: <220.862574439@critter> from Poul-Henning Kamp at "May 2, 97 02:00:39 pm"
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> >Moving items between two lists should be atomic - it is common
> >and lends itself to clean implementation.
> It's actually the same list in this case...
>
> >Aside: how about making the queues truly opaque while you're doing this?
> >Then you have a single place to enable locking hierarchy violation detection,
> >*CAS types of implementations, etc.
>
> I don't think that would be very easy. On the other hand, there
> are very few pieces of code manipulating the vnode free list, do
> it's hardly worth the effort. Or did I misunderstand you ?
I meant this: if I had the time for some research I'd replace all
the *Q* macros with a single opaque data structure (and most likely
a single flavor of queue) with access functions and see what effect
that had on system performance. I'd push operations that leant
themselves to non-locking approaches (e.g., moving between lists)
down there and try to use those. I doubt the performance hit would
be much. Then the locking could be kept down there. Let this
slide - it is idle speculation.
> if (simple_lock_try(&vnode_free_list_slock)) {
I have to look at vtouch now since I don't know that you aren't
committed to do the move to the tail. Can't someone have locked
the freelist for another reason? If you don't have to do the move
it looks good. There is still an ordering, as you know, and I'll
have to find the "white paper" that explains how the lock ordering
is supposed to work.
Anyway, it looks fun. It is good to see this showing up in FBSD.
--
Peter Dufault (dufault@hda.com) Realtime Machine Control and Simulation
HD Associates, Inc. Voice: 508 433 6936
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