Re: MTX Lock implementation question

--- On Wed, 3/4/09, John Baldwin <jhb_at_freebsd.org> wrote:

> From: John Baldwin <jhb_at_freebsd.org>
> Subject: Re: MTX Lock implementation question
> To: barney_cordoba_at_yahoo.com
> Cc: current_at_freebsd.org
> Date: Wednesday, March 4, 2009, 1:05 PM
> On Wednesday 04 March 2009 12:34:40 pm Barney Cordoba wrote:
> > 
> > --- On Wed, 3/4/09, John Baldwin
> <jhb_at_freebsd.org> wrote:
> > 
> > > From: John Baldwin <jhb_at_freebsd.org>
> > > Subject: Re: MTX Lock implementation question
> > > To: freebsd-current_at_freebsd.org,
> barney_cordoba_at_yahoo.com
> > > Cc: current_at_freebsd.org
> > > Date: Wednesday, March 4, 2009, 10:13 AM
> > > On Wednesday 04 March 2009 7:54:32 am Barney
> Cordoba wrote:
> > > > 
> > > > Suppose the following:
> > > > 
> > > > Module foo.c
> > > > 
> > > > foo_getlock(sc)
> > > > {
> > > >    FOO_LOCK(sc);
> > > > }
> > > > foo_unlock(sc)
> > > > {
> > > >    FOO_UNLOCK(sc);
> > > > }
> > > > foo_dosomething(sc)
> > > > {
> > > >    MTX_LOCK_ASSERT(sc);
> > > >    foo_dooit();
> > > > }
> > > > 
> > > > Module bar.c
> > > > 
> > > > bar_dofoo()
> > > > {
> > > >    foo_getlock(sc);
> > > >    foo_dosomething(sc);
> > > >    foo_unlock(sc);
> > > > }
> > > 
> > > This works fine.
> > >  
> > > > Is this something that shouldn't work? I
> need to
> > > access functions
> > > >  that require locks in a different module,
> but this
> > > code barfs on 
> > > > ASSERT with witness enabled. Is this a
> deflugalty in
> > > WITNESS, or do 
> > > > I need to create functions within foo that
> do the
> > > locking? Its been
> > > > working ok for awhile (its not a high volume
> function)
> > > but when I 
> > > > fired up witness to debug something else I
> encountered
> > > witness panics.
> > > 
> > > You probably have a real bug that WITNESS is
> warning about
> > > however.
> > > 
> > > -- 
> > > John Baldwin
> > 
> > It seems that theres a problem when the mutex is
> initialized with a type
> > of NULL. Changing it to a non-null string eliminated
> the issue. I'm
> > running a 7.0 base system.
> 
> Hmm, mutexes are required to be named.  The
> "type" argument is optional, but 
> the name is not.  So you can't do:
> 
> 	mtx_init(&m, NULL, NULL, MTX_DEF);
> 
> 
> You can, however do:
> 
> 	mtx_init(&m, "foo", NULL, MTX_DEF);


There seems to be an issue when the type is set to NULL. I got
panics in strcpy and strlen functions when displaying warning
traces that cleared up when the type was set to a string.

BC