Checking for other kernel modules on load

[Kostik Belousov]

On Thu, Dec 29, 2011 at 11:46:57AM +0000, Chris Rees wrote:
> 2011/12/28 Kostik Belousov <kostikbel at gmail.com>:
> > On Wed, Dec 28, 2011 at 02:53:42PM +0000, Chris Rees wrote:
> >> 2011/12/28 Kostik Belousov <kostikbel at gmail.com>:
> >> > On Wed, Dec 28, 2011 at 12:23:58PM +0000, Chris Rees wrote:
> >> >> On 28 December 2011 12:21, Daniel O'Connor <doconnor at gsoft.com.au> wrote:
> >> >> >
> >> >> > On 28/12/2011, at 22:07, Chris Rees wrote:
> >> >> >> Is there a simple way to check for existence of a driver?  I could
> >> >> >> even check for /dev/sndstat, though that doesn't seem elegant to me...
> >> >> >
> >> >> > kldstat -v, but really /dev/sndstat seems simpler and just as effective.
> >> >> >
> >> >>
> >> >> Cheers-- I was thinking of a kernel-level function though.
> >> >>
> >> >> cognet@ suggested using modfind("sound"), I'll go with that.
> >> > Obvious question is what the panic is. Checking for modules loaded is
> >> > papering over some issue.
> >>
> >> True, although I figured that it was a simple conflict, possibly to do
> >> with sndstat.
> >>
> >> Also, I'm getting panics with the following patch, whether sound is
> >> loaded or not :)
> >>
> >> +  if (modfind("sound") >= 0)
> >> +    {
> >> +      cmn_err (CE_WARN, "A conflicting sound driver is already loaded");
> >> +      return EBUSY;
> >> +    }
> >> +
> >>
> >> Is there a better way to handle such conflicts?
> >
> > You have missed the point. There is some bug in oss driver that causing
> > the panic. Presumed 'conflict' cannot cause the harm itself, besides not
> > allowing second driver to attach to the same device, and should not result
> > in panic. Trying to implement a half-measure that only covers the problem
> > you do a mis-service.
> >
> > And you still did not provided the panic message.
> 
> I'm sorry, you're right.  However, your guess was in fact correct;
> make_dev was being called, which returned a null pointer because it
> failed.
> 
> The patch at [1] stops the panic, however I was hoping that returning
> EBUSY would abort loading the module... At the moment it loads the
> module, and doesn't create the sndstat dev, which causes weird errors
> with the oss binary commands.
> 
> Since this solves the panic and anyone should be able to work out from
> the warning message what the problem is, AND this is a port that
> apparently no-one else uses, should this be sufficient?
> 
> BTW, it only affects FreeBSD 9+, couldn't reproduce on my 8.2 dev
> machine, but could once I upgraded it.
On 8.2, there is no check in the devfs for duplicated cdev names, AFAIR.
So you get absolutely undeterministic behaviour which driver is referenced
by devfs node.

> Chris
> 
> [1] http://www.bayofrum.net/~crees/patches/oss-patch-kernel-OS-FreeBSD-os_freebsd.c

I highly recommend to return error in case of any make_dev_p(9) failure, and
not only EEXIST.
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