Re: witness oddity

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_at_FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org