cvs commit: src/sys/vm vm_fault.c

[Brian Fundakowski Feldman]

On Sat, Aug 21, 2004 at 06:31:34PM -0500, Alan Cox wrote:
> On Sat, Aug 21, 2004 at 06:59:39PM -0400, Brian Fundakowski Feldman wrote:
> > On Sat, Aug 21, 2004 at 07:20:21PM +0000, Alan Cox wrote:
> > > alc         2004-08-21 19:20:21 UTC
> > > 
> > >   FreeBSD src repository
> > > 
> > >   Modified files:
> > >     sys/vm               vm_fault.c 
> > >   Log:
> > >   Further reduce the use of Giant by vm_fault(): Giant is held only when
> > >   manipulating a vnode, e.g., calling vput().  This reduces contention for
> > >   Giant during many copy-on-write faults, resulting in some additional
> > >   speedup on SMPs.
> > >   
> > >   Note: debug_mpsafevm must be enabled for this optimization to take effect.
> > 
> > This is very broken.  See included first attempt at fixing it without
> > regard for actually trying to reimplement debug.mpsafenet for vnodes.
> > 
> 
> Can you please explain what is broken?

#1. Lock order reversal.  Giant is acquired after the map read lock.
#2. Unlocking Giant without it being locked.  This occurs in both
    VM_UNLOCK_GIANT() lines I removed.  I know this because my machine
    panicked on the other after I removed the first.

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