[PATCH] drm/atomic: Paper over locking WARN in default_state_clear

[Daniel Vetter]

On Fri, Jul 31, 2015 at 10:34:43AM +0200, Maarten Lankhorst wrote:
> Hey,
> 
> Op 29-07-15 om 12:51 schreef Daniel Vetter:
> > In
> >
> > commit 6f75cea66c8dd043ced282016b21a639af176642
> > Author: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Date:   Wed Nov 19 18:38:07 2014 +0100
> >
> >     drm/atomic: Only destroy connector states with connection mutex held
> >
> > I tried to fix races of atomic commits against connector
> > hot-unplugging. The idea is to ensure lifetimes by holding the
> > connection_mutex long enough. That works for synchronous commits, but
> > not for async ones.
> >
> > For async atomic commit we really need to fix up connector lifetimes
> > for real. But that's a much bigger task, so just add more duct-tape:
> > For cleaning up connector states we currently don't need the connector
> > itself. So NULL it out and remove the locking check. Of course that
> > check was to protect the entire sequence, but the modeset itself
> > should be save since currently DP MST hot-removal does a dpms-off. And
> > that should synchronize with any outstanding async atomic commit.
> >
> > Or at least that's my hope, this is all a giant mess.
> >
> > Reported-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  drivers/gpu/drm/drm_atomic.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> > index 3efd91c0c6cb..434915448ea0 100644
> > --- a/drivers/gpu/drm/drm_atomic.c
> > +++ b/drivers/gpu/drm/drm_atomic.c
> > @@ -153,9 +153,15 @@ void drm_atomic_state_default_clear(struct drm_atomic_state *state)
> >  		if (!connector)
> >  			continue;
> >  
> > -		WARN_ON(!drm_modeset_is_locked(&config->connection_mutex));
> > -
> > -		connector->funcs->atomic_destroy_state(connector,
> > +		/*
> > +		 * FIXME: Async commits can race with connector unplugging and
> > +		 * there's currently nothing that prevents cleanup up state for
> > +		 * deleted connectors. As long as the callback doesn't look at
> > +		 * the connector we'll be fine though, so make sure that's the
> > +		 * case by setting all connector pointers to NULL.
> > +		 */
> > +		state->connector_states[i]->connector = NULL;
> > +		connector->funcs->atomic_destroy_state(NULL,
> >  						       state->connector_states[i]);
> >
> This wouldn't provide any additional guarantee during the async commit
> itself, so please don't do this. :-)

Nope, it's really just a big reminder that we have a bug here.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch