Date: Fri, 10 Sep 2004 16:29:32 -0400 From: John Baldwin <jhb@FreeBSD.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-current@FreeBSD.org Subject: Re: witness oddity Message-ID: <200409101629.32653.jhb@FreeBSD.org> In-Reply-To: <16706.309.398789.905433@grasshopper.cs.duke.edu> References: <16705.57806.550902.483858@grasshopper.cs.duke.edu> <200409101450.47478.jhb@FreeBSD.org> <16706.309.398789.905433@grasshopper.cs.duke.edu>
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On Friday 10 September 2004 03:32 pm, Andrew Gallatin wrote:
> John Baldwin writes:
> > On Friday 10 September 2004 02:18 pm, Andrew Gallatin wrote:
> > > John-Mark Gurney writes:
> > > > Andrew Gallatin wrote this message on Fri, Sep 10, 2004 at 13:18
-0400:
> > > > > If I call copyout() holding one of my mutexes, it will always
> > > > > complain about a LOR, even if the mutex is freshly initiated:
> > > >
> > > > Calling copyout while holding a mutex is not allowed... If the
> > > > page isn't in memory, it could take many seconds for the page to be
> > > > swapped back in during which time your mutex will continue to be
> > > > held.
> > >
> > > Thanks.. but that's not really what I asked.
> > >
> > > I want to know how witness detects a particular just-created mutex as
> > > being in a deadlock with the vm map lock.
> > >
> > > Again, is it because the vm lock is an sx lock? Is there an implicit
> > > rule that you can't take an sx lock while holding a mutex (just like
> > > you can't take Giant, or sleep?)
> >
> > Yes. An sx lock is allowed to be held across a sleep, so if you block
> > on an sx lock, the owner of the lock you are waiting on might be asleep.
> > If that
>
> Do you agree that the message that Witness emits ("lock order
> reversal") for this problem is, while technically accurate, is at
> least a little confusing? Before I thought to try the
> mtx_init()/mtx_lock/()/copyout() trick, I spent quite a while scanning
> my code, looking for some way the VM system could call into it and
> acquire that lock. There aren't any.
>
> Does witness know at the time that it emits the warning that its a
> "class" type of reversal, rather than a reversal based on previous
> observations? If so, would it be possible to emit a warning saying
> something like "Holding a sleep mutex while acquiring an sx lock is
> probited by law" (maybe add " violators will be shot" for grins ;)
That's a possibility yes.
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve" = http://www.FreeBSD.org
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