Date: Wed, 22 Jan 2014 16:22:42 -0500 From: John Baldwin <jhb@freebsd.org> To: Alfred Perlstein <alfred@freebsd.org> Cc: src-committers@freebsd.org, Scott Long <scott4long@yahoo.com>, Neel Natu <neel@freebsd.org>, John-Mark Gurney <jmg@funkthat.com>, svn-src-all@freebsd.org, Rui Paulo <rpaulo@felyko.com>, svn-src-head@freebsd.org, Alexander Kabaev <kabaev@gmail.com> Subject: Re: svn commit: r260898 - head/sys/kern Message-ID: <201401221622.42789.jhb@freebsd.org> In-Reply-To: <52E03139.2020902@freebsd.org> References: <201401200159.s0K1xa5X012123@svn.freebsd.org> <201401221527.12779.jhb@freebsd.org> <52E03139.2020902@freebsd.org>
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On Wednesday, January 22, 2014 3:59:37 pm Alfred Perlstein wrote:
>
> On 1/22/14, 12:27 PM, John Baldwin wrote:
> > On Wednesday, January 22, 2014 2:06:39 pm Alfred Perlstein wrote:
> >> Hmm, what if locks had a pointer to a 2 element char * array, the first
> >> being the name, the second the type. That would keep the size of the
> >> lock down and most locks could share a common tuple of name/type in each
> >> subsystem. This would allow us to get rid of the pending static list.
> >>
> >> effectively:
> >> struct lock_object {
> >> char *lo_name; /* Individual lock name. */
> >> u_int lo_flags;
> >> u_int lo_data; /* General class specific data.
*/
> >> struct witness *lo_witness; /* Data for witness. */
> >> };
> >>
> >> would change to:
> >> struct lock_object {
> >> char **lo_name_type; /* Individual lock
> >> name[0]/type[1]. */
> >> u_int lo_flags;
> >> u_int lo_data; /* General class specific data.
*/
> >> struct witness *lo_witness; /* Data for witness. */
> >> };
> >>
> >> This may be somewhat disruptive, I haven't played with how it would
> >> actually change driver/etc/code.
> > Where would the memory for the char* array come from?
> >
> That is a good question. I suspect it would be up to the subsystem to
> allocate it.
>
> Wouldn't it be trivial for *most* of the subsystems to simply have this
> either as a static global or static function variable:
>
> static char *mutex_typename = { "kqueue", "foo" };
>
> Under kern I see this:
> grep mtx_init * | grep -v NULL
> ...
> kern_rmlock.c: mtx_init(&rm->rm_lock_mtx, name, "rmlock_mtx",
> MTX_NOWITNESS);
> subr_bus.c: mtx_init(&devsoftc.mtx, "dev mtx", "devd", MTX_DEF);
>
> Those are solved with statics.
>
> Another example:
>
> sys/dev/ae/if_ae.c
> mtx_init(&sc->mtx, device_get_nameunit(dev), MTX_NETWORK_LOCK,
> MTX_DEF);
>
> I think the array could be in the softc here? sc->mutex_name_type[0] =
> device_get_nameunit(dev); sc->mutex_name_type[1] = MTX_NETWORK_LOCK;
>
> Do we want to do that? It moves "wasting space" to another variable.
>
> I'm not sure where there isn't the possibility of using either static
> (for a global mutex) or space inside the equiv of the softc (or proc or
> whatever) for this?
>
> I'm not sure this is a good idea, just an idea. Are there places where
> it's not as simple as doing this?
To be honest, the whole name vs type thing isn't widely used, and it makes
the mtx_init() function kind of fugly. I think what I would actually prefer
is to just kill it, changing the various places that pass a separate name to
just pass the type instead. Note that none of the other lock APIs even allow
setting a separate type. This would let us remove the static pending list
array as well.
(And yes, I added the name vs type thing, but at this point I think it did
not turn out nearly as useful as I had thought it would be.)
The original issue of picking useful-to-witness lock names (i.e. not just
using device_get_nameunit()) still remains of course.
--
John Baldwin
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