Date: Tue, 05 Jan 1999 17:00:23 -0700 From: Wes Peters <wes@softweyr.com> To: Nate Williams <nate@mt.sri.com> Cc: Terry Lambert <tlambert@primenet.com>, narvi@haldjas.folklore.ee, bright@hotjobs.com, hackers@FreeBSD.ORG Subject: Re: question about re-entrancy. Message-ID: <3692A797.975E8433@softweyr.com> References: <199901052008.NAA09332@mt.sri.com> <199901052245.PAA24981@usr02.primenet.com> <199901052249.PAA10421@mt.sri.com> <3692A39C.889BF032@softweyr.com> <199901052348.QAA10896@mt.sri.com>
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Nate Williams wrote:
>
> > The problem with this model, which is commonly used, is that I hold the
> > read-lock on the first device while waiting to acquire the write-lock
> > on the second, or vice versa. A parallel waiting system, in which another
> > potential reader of the first device can "take away" my lock if I'm still
> > pending for another resource, alleviates this problem. You have to avoid
> > priority problems when doing this, but inherited priorities are a pretty
> > well understood solution to this problem.
>
> One of the more common ways I've seen this done is through a 'resource
> manager' inside of the kernel which guarantees that you 'lock down' the
> resources in exactly the same order. Unfortunately, this tends to be a
> bottleneck on highly parallel systems. ;(
I guess your "resource manager" *is* my "parallel waiting system." I've
only looked at this in a deeply embedded context, where the 3 processors
share a single coherent cache and transaction-oriented memory bus, so my
experience may not be completely scalable. I should add that it's not
working yet either. ;^)
The working idea is that if I have acquired a lock on one of a set of
objects, but not all of the objects (therefore I'm sleeping), a higher
priority task can 'steal back' an object from me. While quite flexible,
it requires *very careful attention* to priorities, and opens up avenues
to deadlocks that boggle the mind. In our particular application, it
is quite a neat little trick, where we have three processors and what
amounts to seven input queues and seven output queues, and need to route
data between 1..7 of the queues at a time.
--
Where am I, and what am I doing in this handbasket?
Wes Peters +1.801.915.2061
Softweyr LLC wes@softweyr.com
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