cvs commit: src/sys/vm vm_fault.c

[Brian Fundakowski Feldman]

On Sun, Aug 22, 2004 at 12:27:31AM -0500, Alan Cox wrote:
> On Sun, Aug 22, 2004 at 01:01:55AM -0400, Brian Fundakowski Feldman wrote:
> > On Sat, Aug 21, 2004 at 11:49:36PM -0500, Alan Cox wrote:
> > > On Sun, Aug 22, 2004 at 12:18:49AM -0400, Robert Watson wrote:
> > > > 
> > > > On Sun, 22 Aug 2004, Brian Fundakowski Feldman wrote:
> > > > 
> > > > > Also, it was the system_map lock, so it was a mutex, not an sx. 
> > > > > 
> > > > > /Goes back to trying to figure out wtf portupgrade -rR kde\* keeps
> > > > > causing
> > > > >  hangs, but only in X, not at the command line, and not seemingly
> > > > >  dependant on AGP/X driver. 
> > > > 
> > > > I believe mutexes are alright also as long as you're careful about
> > > > sleeping and lock order.  I.e., don't sleep while holding a
> > > > non-funnel-like mutex, etc.
> > > > 
> > > 
> > > We should never page fault on a system map.  If that happens to you,
> > > it indicates an error.  (In-kernel maps on which we do page fault,
> > > such as the pipe submap, are treated the same as user maps and utilize
> > > an sx lock.)
> > 
> > vm_fault
> > vm_fault_wire
> > vm_map_wire
> > kmem_alloc
> > vm_ksubmap_init
> > cpu_startup
> > mi_startup
> > 
> 
> This is a call to vm_fault(), but not a page fault.  This works
> without error because kmem_alloc() has preallocated and wired all of
> the required pages before calling vm_map_wire() (which calls
> vm_fault()).  Specifically, the preallocation and wiring guarantees
> that vm_fault() will not follow any of the code paths on which it
> could sleep with the system map mutex held.  (In fact, I believe that
> we are guaranteed that it will not sleep under any circumstances.)

I know, but that has nothing to do with the fact that it's locking the
system map mutex, then Giant, which causes a lock order reversal.  Do
you run WITNESS?  If you do, you should see this in your startup.

-- 
Brian Fundakowski Feldman                           \'[ FreeBSD ]''''''''''\
  <> green at FreeBSD.org                               \  The Power to Serve! \
 Opinions expressed are my own.                       \,,,,,,,,,,,,,,,,,,,,,,\