Date: Mon, 28 Dec 2015 12:51:57 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Daniel Eischen <deischen@freebsd.org> Cc: John Baldwin <jhb@freebsd.org>, freebsd-arch@freebsd.org, freebsd-threads@freebsd.org Subject: Re: libthr shared locks Message-ID: <20151228105157.GQ3625@kib.kiev.ua> In-Reply-To: <Pine.GSO.4.64.1512271110450.23305@sea.ntplx.net> References: <4199356.DlQeWDh27F@ralph.baldwin.cx> <Pine.GSO.4.64.1512240836460.9582@sea.ntplx.net> <5496837.TbTQtANDNj@ralph.baldwin.cx> <Pine.GSO.4.64.1512241338340.10649@sea.ntplx.net> <20151224191408.GA3625@kib.kiev.ua> <Pine.GSO.4.64.1512251310090.14902@sea.ntplx.net> <20151226105409.GH3625@kib.kiev.ua> <Pine.GSO.4.64.1512261151550.19056@sea.ntplx.net> <20151226234424.GJ3625@kib.kiev.ua> <Pine.GSO.4.64.1512271110450.23305@sea.ntplx.net>
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On Sun, Dec 27, 2015 at 11:44:44AM -0500, Daniel Eischen wrote:
> On Sun, 27 Dec 2015, Konstantin Belousov wrote:
>
> > On Sat, Dec 26, 2015 at 12:15:43PM -0500, Daniel Eischen wrote:
> [ snipped for brevity ]
> >>
> >> I agree, but the work that you are doing now would be basically
> >> thrown out later on. I will not stand in your way and appreciate
> >> any work you do. I would just rather that the struct change be
> >> made now for 11, even without any pshared or other changes. For
> >> once the struct change is made, pshared or other additions can
> >> be made afterward, even in the 11 branch because they would not
> >> break the ABI.
> >
> > Lock inlining was not done for ten years, now cost of doing it is
> > extremely high, as discussed above. Who would drive the change, and
> > with what time frame ? If me, I seriosly consider renaming libthr
> > to libthr2, but I had no time to think much about it.
>
> I could probably do the inlining of locks, pulling out that part
> of it from David's patch, and coordinating with you so we don't
> step on each other's toes. Perhaps just making the first element
> of the structs a self-reference at first, would help mitigate
> that...
Adding the self-pointer is good idea, but it would not work for the
shared locks. If you are going to work on this, then I will scratch
my patch, to not impede on your work.
Taking out the inlining bits from the David patch, or (which would I do,
if doing this) just reimplementing it from scratch is easy enough and
just require some time. I estimated this job to take between one and
two weeks.
What is really thrilling is to manage the consequences of the ABI
breakage. When I did the evaluation for the FF project, I put a six
months extent for the whole work. This is for things like looking at
the ports impact, trying to know in advance where mixed things start to
break, monitoring the users complains about issues caused by the ABI
break etc.
We would see afterward if I overestimated the work, but I do the typical
mistake of underestimating usually, whatever insane large the initial
numbers are.
>
> I'm not sure what renaming libthr to libthr2 solves that a
> version bump can't also. Can't we still tell whether both
> libthr.so.3 and libthr.so.4 have been loaded? Perhaps libthr2
> is cleaner WRT keeping the old ABI (it could be dropped?).
Yes, part of the change probably should be a prevention of simultaneous
existence of libthr.so.3 and (libthr.so.4 or libthr2.so.1, whatever
it is named) in one process. This should be done either with rtld
facilities, I am not sure how. Or e.g. with dlopen("otherlib",
RTLD_NOLOAD) in the constructor of each library and failing if dlopen()
returns a valid handle.
Another immediate point, not only libthr.so must be bumped, but also all
base libraries depending on it must be. Even if some libraries do not
directly record the dependency, they might require the bump as well, I
am thinking about c++ runtime. This should allow the compat packages
to provide useable libraries.
IMO this is easier to see when the libraries names differ significantly
in the name part, and not in the .so.n part. At least users would be
less surprised, but also the work of the person who tracks the deps,
would be easier too.
>
> > Right now, I think that I want to commit my current patch and implement
> > robust mutexes as the next step, without ABI breakage. At least, this
> > seems to have fixed time-frame and can be made ready for 11.x. Lock
> > inlining might be not. Are there serious objections against the plan,
> > except that (lock inlining + pshared) is ideal situation, while the plan
> > is not (but more practical) ?
>
> What is the timeframe for 11.0-RELEASE? If not for 11.0, I would
> like to see it done soon in the 12-current branch afterward.
> In my mind, any pain will be the same in 11 or 12, nothing really
> is gained by waiting, only by not doing it (inlining locks) ever.
Difference is in causing more (ABI) breakage on the near-coming
stable/11 users, or trying to fix or mitigate more of it in HEAD,
using the local population as the canaries.
You see my position, I would avoid ABI breakage at all costs. If I
cannot talk you against this, please do not consider the work done after
the inlining patch is committed to HEAD, it is actually only start
there.
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