Re: Tracking down LORs

On Friday 20 August 2004 04:08 am, Roman Kurakin wrote:
> Robert Watson wrote:
> >On Fri, 20 Aug 2004, Roman Kurakin wrote:
> >>	Currently I am trying to track down a couple of LORS in my code.
> >>But it seems that I do not undestand smth or all things id realy so bad.
> >
> >I find it's very helpful to add lock orders to the hard-coded lock order
> >table in subr_witness.c.  Without hard-coded entries, WITNESS will
> >dynamically build an order based on observed lock use.  This is generally
> >fine, but once in a while the "wrong" order will be used before the
> >"right" order, so the lock order warning will print for the "right" order,
> >leaving less useful debugging information.  The table allows the
> >definition of partial orders, so you can specify relationships between
> >subsets of mutexes of interest.  WITNESS will flesh out remaining orders
> >through dynamic discovery.
>
> I'll try to go this way, since I am in dead end.

Note that some lock orders as also inferred via transitivity.

For example, let's say you have three locks a, b, and c.  One thread locks a 
then b, so witness saves that order.  A second thread locks b then c, so 
witness saves that order.  If a third thread tries to lock c then a, an LOR 
will result.  This is similar to how > is transitive, i.e. if a > b and b > 
c, then a > c.

-- 
John Baldwin <jhb_at_FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org