2007/11/24, Attilio Rao <attilio freebsd.org>:
> 2007/11/24, Robert Watson <rwatsonfreebsd.org>:
> >
> > On Fri, 23 Nov 2007, Stephan Uphoff wrote:
> >
> > >>> I talked with Attilio about that on
IRC. Most common cases of writer
> > >>> starvation (but not all) could be
solved by keeping a per thread count of
> > >>> shared acquired rwlocks. If a rwlock
is currently locked as shared/read
> > >>> AND a thread is blocked on it to lock
it exclusively/write - then new
> > >>> shared/read locks will only be
granted to thread that already has a shared
> > >>> lock. (per thread shared counter is
non zero)
> > >>>
> > >>> To be honest I am a bit twitchy about
a lock without priority propagation
> > >>> - especially since in FreeBSD threads
run with user priority in kernel
> > >>> space and can get preempted.
> > >>
> > >> That's an interesting hack, I guess it
could be done.
> > >>
> > >> I would still like to disallow
recursion.
> > >>
> > > Oh - I am all for disallowing recursion. In
my opinion the only valid place
> > > for a thread to acquire the same lock
multiple times is inside a transaction
> > > system with full deadlock detection. The
question is if we can do that this
> > > late in the game? Maybe we could make non
recursive the default and add a
> > > call rw_allow_recurse or rw_init_recurse to
allow recursion on a lock if we
> > > can't get away with the straight out removal
of the option? (Or less
> > > desirable - keep the current default and add
functions to disallow
> > > recursion)
> >
> > While I'm no great fan of recursion, the reality
is that many of our kernel
> > subsystems are not yet ready to disallow recursion
on locks. Take a look at
> > the cases where we explicitly enable recursive
acquisition for mutexes--in
> > practice, most network stack mutexes are recursive
due to the recursive
> > calling in the network stack. While someday I'd
like to think we'll be able
> > to eliminate some of that, but it won't be soon
since it requires significant
> > reworking of very complicated code. The current
model in which recursion is
> > explicitly enabled only where still required seems
to work pretty well for the
> > existing code, although it's hard to say yet in
the code I've looked at
> > whether read recursion would be required--the
situations I have in mind would
> > require purely write recursion. There's one case
in the UNIX domain socket
> > code where we do a locked test and conditional
lock/unlock with an rwlock for
> > exclusive locking because recursion isn't
currently supported, and that's not
> > a usage I'd like to encourage more of.
>
> I think however that Stephan is just referring to the
readers
> recursion (as we are doing) that if present should be
just in few
> points.
> If we want todo a radical switch of this genre this is
the right
> moment, just before rwlock became too widespread.
>
> I have a patch against witness which would check for
readers recursion
> and panic, I will post in the night or tomorrow.
So, this patch checks for recursive readers and will panic
in case:
http://people.freebsd.org/~attilio/recursion_checks.diff
(I preferred to do in this way, but you can check for
perforce commit
number 129792 for an alternative way to achieve the same).
What I'm looking for now is for people which can apply this
patch,
enable WITNESS with all the relevant options on and try to
see if
there are recursive readers in rwlocks somewhere (hopefully
not after
the pfil(9) switching to rmlock that mlaier did recently).
Attilio
--
Peace can only be achieved by understanding - A. Einstein
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