[PATCH] drm/drm_bridge: adjust bridge module's refcount

[Daniel Vetter]

On Wed, Nov 30, 2016 at 11:56:25AM +0200, Laurent Pinchart wrote:
> Hi Daniel,
> 
> On Wednesday 30 Nov 2016 10:12:01 Daniel Vetter wrote:
> > On Wed, Nov 30, 2016 at 10:25:57AM +0200, Laurent Pinchart wrote:
> > > On Wednesday 30 Nov 2016 09:13:00 Daniel Vetter wrote:
> > >> On Wed, Nov 30, 2016 at 12:06:25AM +0200, Laurent Pinchart wrote:
> > >>> On Tuesday 29 Nov 2016 23:12:56 Jyri Sarha wrote:
> > >>>> Store the module that provides the bridge and adjust its refcount
> > >>>> accordingly. The bridge module unload should not be allowed while the
> > >>>> bridge is attached.
> > >>>> 
> > >>>> Signed-off-by: Jyri Sarha <jsarha at ti.com>
> > >>>> ---
> > >>>> 
> > >>>>  drivers/gpu/drm/drm_bridge.c | 15 ++++++++++++---
> > >>>>  include/drm/drm_bridge.h     | 11 ++++++++++-
> > >>>>  2 files changed, 22 insertions(+), 4 deletions(-)
> > >>>> 
> > >>>> diff --git a/drivers/gpu/drm/drm_bridge.c
> > >>>> b/drivers/gpu/drm/drm_bridge.c
> > >>>> index 0ee052b..36d427b 100644
> > >>>> --- a/drivers/gpu/drm/drm_bridge.c
> > >>>> +++ b/drivers/gpu/drm/drm_bridge.c
> > > 
> > > [snip]
> > > 
> > >>>> @@ -114,6 +118,9 @@ int drm_bridge_attach(struct drm_device *dev,
> > >>>> struct drm_bridge *bridge)
> > >>>>  	if (bridge->dev)
> > >>>>  		return -EBUSY;
> > >>>> 
> > >>>> +	if (!try_module_get(bridge->module))
> > >>>> +		return -ENOENT;
> > >>> 
> > >>> Isn't this still racy ? What happens if the module is removed right
> > >>> before this call ? Won't the bridge object be freed, and this code then
> > >>> try to call try_module_get() on freed memory ?
> > >>> 
> > >>> To fix this properly I think we need to make the bridge object
> > >>> refcounted, with a release callback to signal to the bridge driver that
> > >>> memory can be freed. The refcount should be increased in
> > >>> of_drm_find_bridge(), and decreased in a new drm_bridge_put() function
> > >>> (the "fun" part will be to update drivers to call that :-S).
> > >>> 
> > >>> The module refcount still needs to be increased in drm_bridge_attach()
> > >>> like you do here, but you'll need to protect it with bridge_lock to
> > >>> avoid a race between try_module_get() and drm_bridge_remove().
> > >> 
> > >> Hm right, I thought _attach is the function called directly, but we do
> > >> lookup+attach. Might be good to have an of helper which does the
> > >> lookup+attach and then drops the reference. We can do that as long as
> > >> attach/detach holds onto a reference like the one below.
> > >> 
> > >> And I don't think we need a new bridge refcount, we can just us
> > >> try_module_get/put for that (but wrapped into helpers ofc).
> > > 
> > > The bridge refcount and the module refcount serve two different purposes.
> > > The first one addresses the unbind race by preventing the object from
> > > being freed while still referenced, to avoid faulty data memory accesses
> > > (note that increasing a module refcount doesn't prevent unbinding the
> > > module from the device). The second one addresses the module unload race
> > > by preventing code from being unloaded while still being reachable
> > > through function pointers, avoiding faulty text memory accesses (as well
> > > as data memory accesses to any .data or .rodata section in the module, if
> > > those are referenced through pointers in the bridge object).
> > > 
> > > We thus need both types of refcounting, with the former tied to the lookup
> > > operation and the latter to the attach operation (even though we could
> > > handle module refcounting at lookup time as well if preferred, but in a
> > > well-behaved system the bridge callbacks should not be invoked before
> > > attach time).
> >
> > So you're saying struct drm_bridge should embed a struct device? Or at
> > least kobj? Handling that race is one of the reasons we have them ...
> 
> I'm thinking about kref + a release callback. The really fun part to solve 
> this in the long term will be to teach the DRM core about encoder hot-unplug 
> :-) In the meantime the minimum we need is to allow the system to fail safely 
> when a bridge is unplugged, which will involve safe recovery from 
> .atomic_commit() failures.

Why exactly do you want to hotplug encoders? Or bridges fwiw ... since at
least only making those hotpluggable will make the uabi story easier since
they're not exposed.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch