[pulseaudio-discuss] [PATCH v5 39/39] bluetooth: Revive module-bluetooth-discover

[Tanu Kaskinen]

On Wed, 2013-10-09 at 12:26 -0300, João Paulo Rechi Vita wrote:
> On Tue, Oct 8, 2013 at 12:15 PM, Tanu Kaskinen
> <tanu.kaskinen at linux.intel.com> wrote:
> > On Tue, 2013-10-08 at 11:19 -0300, João Paulo Rechi Vita wrote:
> >> On Sun, Sep 29, 2013 at 1:39 PM, Tanu Kaskinen
> >> <tanu.kaskinen at linux.intel.com> wrote:
> >> > On Tue, 2013-09-24 at 19:45 -0300, jprvita at gmail.com wrote:
> >> >> +void pa__done(pa_module* m) {
> >> >> +    struct userdata *u;
> >> >> +
> >> >> +    pa_assert(m);
> >> >> +
> >> >> +    if (!(u = m->userdata))
> >> >> +        return;
> >> >> +
> >> >> +    if (u->bluez5_module)
> >> >> +        pa_module_unload(m->core, u->bluez5_module, true);
> >> >> +
> >> >> +    if (u->bluez4_module)
> >> >> +        pa_module_unload(m->core, u->bluez4_module, true);
> >> >
> >> > This crashes when shutting down the daemon, because when the daemon
> >> > unloads all modules, module-bluez*-discover gets unloaded before
> >> > module-bluetooth-discover, so the y->bluez5_module and u->bluez4_module
> >> > pointers become stale. I see two ways of fixing this: add a hook that is
> >> > fired when modules are unloaded and use that hook in
> >> > module-bluetooth-discover to drop the reference to the unloaded module,
> >> > or unload module-bluetooth-discover immediately after loading
> >> > module-bluez5-discover and module-bluez4-discover. The second solution
> >> > is of course much simpler, but I proposed that already earlier, and you
> >> > didn't like that.
> >> >
> >>
> >> It doesn't crash (and that's what I'm experiencing here) because
> >> pa_module_unload() will look for that module pointer in its internal
> >> hash of modules before trying to unload it. I agree we are left with a
> >> stale pointer, but as long as we don't dereference it, we should be
> >> fine.
> >
> > pa_module_unload() dereferences the pointer already before
> >
> >     if (!(m = pa_idxset_remove_by_data(c->modules, m, NULL)))
> >         return;
> >
> > which I think you are referring to as the safeguard. The line that
> > crashes is this:
> >
> >     if (m->core->disallow_module_loading && !force)
> >
> > It doesn't crash always, because m->core is a random address that may or
> > may not be possible to dereference (it was 0 when I debugged this).
> >
> > When this line doesn't crash, I get an assertion about pa_core.shared
> > not being empty when pa_core is freed. I hope that's another symptom of
> > this same bug (I looked at the code and the management of pa_core.shared
> > seemed correct).
> >
> 
> I don't get the assertion either. I'm testing with the current master,
> which has this patch included.
> 
> Anyway, I guess one solution would be to keep track of
> module-bluez[45]-discover using the module index instead of the module
> pointer (assuming indexes are not reused in a same pulseaudio run).
> This way the caller would pass the pa_core pointer in the call instead
> of relying on the module pointer. What do you think?

I don't know what you mean by "the caller would pass the pa_core pointer
in the call instead of relying on the module pointer". I understand your
proposal so that you'd use pa_module_unload_by_index() instead of
pa_module_unload(). That should work, and I'm fine with that solution.

-- 
Tanu