Re: [PATCH v3 05/10] drm/fb-helper: Add top-level lock

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On Tue, Mar 21, 2017 at 11:00:23AM +0100, Daniel Vetter wrote:
> On Tue, Mar 21, 2017 at 09:13:53AM +0100, Thierry Reding wrote:
> > From: Thierry Reding <treding@xxxxxxxxxx>
> > 
> > Introduce a new top-level lock for the FB helper code. This will allow
> > better locking granularity and avoid the need to abuse modeset locking
> > for this purpose instead.
> > 
> > Tested-by: John Stultz <john.stultz@xxxxxxxxxx>
> > Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
> 
> Some suggestions below, since I think this state of locking implemented
> here is a bit a too awkwared intermediate thing.

To clarify: After having reviewed the next patch I think this here is
fine. So gets an Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
as-is. I just think we want a notch more here.

But if you're burried too much we can do that as a follow-up too, but in
that case please add a FIXME to e.g. the @lock kernel-doc:

"FIXME: fbdev emulation locking is a mess, long term we want to protect
all helper internal state with this lock, and reduce core KMS locking as
much as possible."

Your choice :-)

Cheers, Daniel
> 
> > ---
> >  drivers/gpu/drm/drm_fb_helper.c | 27 ++++++++++++++++++++++++++-
> >  include/drm/drm_fb_helper.h     |  7 +++++++
> >  2 files changed, 33 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> > index 6af80afc4f40..9060adcf7cf8 100644
> > --- a/drivers/gpu/drm/drm_fb_helper.c
> > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > @@ -119,6 +119,7 @@ static int __drm_fb_helper_add_one_connector(struct drm_fb_helper *fb_helper,
> >  	if (!drm_fbdev_emulation)
> >  		return 0;
> >  
> > +	WARN_ON(!mutex_is_locked(&fb_helper->lock));
> >  	WARN_ON(!mutex_is_locked(&fb_helper->dev->mode_config.mutex));
> 
> mode_config.mutex isn't needed anymore here (with the recommendations at
> the bottom), please remove.
> >  
> >  	count = fb_helper->connector_count + 1;
> > @@ -141,6 +142,7 @@ static int __drm_fb_helper_add_one_connector(struct drm_fb_helper *fb_helper,
> >  	drm_connector_get(connector);
> >  	conn->connector = connector;
> >  	fb_helper->connector_info[fb_helper->connector_count++] = conn;
> > +
> >  	return 0;
> >  }
> >  
> > @@ -149,11 +151,13 @@ int drm_fb_helper_add_one_connector(struct drm_fb_helper *fb_helper,
> >  {
> >  	int err;
> >  
> > +	mutex_lock(&fb_helper->lock);
> >  	mutex_lock(&fb_helper->dev->mode_config.mutex);
> 
> Same.
> 
> >  
> >  	err = __drm_fb_helper_add_one_connector(fb_helper, connector);
> >  
> >  	mutex_unlock(&fb_helper->dev->mode_config.mutex);
> > +	mutex_unlock(&fb_helper->lock);
> >  
> >  	return err;
> >  }
> > @@ -183,6 +187,7 @@ int drm_fb_helper_single_add_all_connectors(struct drm_fb_helper *fb_helper)
> >  	if (!drm_fbdev_emulation)
> >  		return 0;
> >  
> > +	mutex_lock(&fb_helper->lock);
> >  	mutex_lock(&dev->mode_config.mutex);
> 
> Same.
> 
> >  	drm_connector_list_iter_begin(dev, &conn_iter);
> >  	drm_for_each_connector_iter(connector, &conn_iter) {
> > @@ -206,6 +211,7 @@ int drm_fb_helper_single_add_all_connectors(struct drm_fb_helper *fb_helper)
> >  out:
> >  	drm_connector_list_iter_end(&conn_iter);
> >  	mutex_unlock(&dev->mode_config.mutex);
> > +	mutex_unlock(&fb_helper->lock);
> >  
> >  	return ret;
> >  }
> > @@ -220,6 +226,7 @@ static int __drm_fb_helper_remove_one_connector(struct drm_fb_helper *fb_helper,
> >  	if (!drm_fbdev_emulation)
> >  		return 0;
> >  
> > +	WARN_ON(!mutex_is_locked(&fb_helper->lock));
> >  	WARN_ON(!mutex_is_locked(&fb_helper->dev->mode_config.mutex));
> 
> Same.
> 
> >  
> >  	for (i = 0; i < fb_helper->connector_count; i++) {
> > @@ -246,11 +253,13 @@ int drm_fb_helper_remove_one_connector(struct drm_fb_helper *fb_helper,
> >  {
> >  	int err;
> >  
> > +	mutex_lock(&fb_helper->lock);
> >  	mutex_lock(&fb_helper->dev->mode_config.mutex);
> >  
> >  	err = __drm_fb_helper_remove_one_connector(fb_helper, connector);
> >  
> >  	mutex_unlock(&fb_helper->dev->mode_config.mutex);
> > +	mutex_unlock(&fb_helper->lock);
> >  
> >  	return err;
> >  }
> > @@ -502,16 +511,21 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
> >  	if (!drm_fbdev_emulation)
> >  		return -ENODEV;
> >  
> > +	mutex_lock(&fb_helper->lock);
> >  	drm_modeset_lock_all(dev);
> > +
> >  	ret = restore_fbdev_mode(fb_helper);
> >  
> >  	do_delayed = fb_helper->delayed_hotplug;
> >  	if (do_delayed)
> >  		fb_helper->delayed_hotplug = false;
> > +
> >  	drm_modeset_unlock_all(dev);
> > +	mutex_unlock(&fb_helper->lock);
> >  
> >  	if (do_delayed)
> >  		drm_fb_helper_hotplug_event(fb_helper);
> > +
> >  	return ret;
> >  }
> >  EXPORT_SYMBOL(drm_fb_helper_restore_fbdev_mode_unlocked);
> > @@ -747,6 +761,7 @@ void drm_fb_helper_prepare(struct drm_device *dev, struct drm_fb_helper *helper,
> >  	INIT_WORK(&helper->resume_work, drm_fb_helper_resume_worker);
> >  	INIT_WORK(&helper->dirty_work, drm_fb_helper_dirty_work);
> >  	helper->dirty_clip.x1 = helper->dirty_clip.y1 = ~0;
> > +	mutex_init(&helper->lock);
> >  	helper->funcs = funcs;
> >  	helper->dev = dev;
> >  }
> > @@ -912,6 +927,7 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
> >  	}
> >  	mutex_unlock(&kernel_fb_helper_lock);
> >  
> > +	mutex_destroy(&fb_helper->lock);
> >  	drm_fb_helper_crtc_free(fb_helper);
> >  
> >  }
> > @@ -2421,25 +2437,34 @@ EXPORT_SYMBOL(drm_fb_helper_initial_config);
> >  int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
> >  {
> >  	struct drm_device *dev = fb_helper->dev;
> > +	int err = 0;
> >  
> >  	if (!drm_fbdev_emulation)
> >  		return 0;
> >  
> > +	mutex_lock(&fb_helper->lock);
> >  	mutex_lock(&dev->mode_config.mutex);
> > +
> >  	if (!fb_helper->fb || !drm_fb_helper_is_bound(fb_helper)) {
> >  		fb_helper->delayed_hotplug = true;
> >  		mutex_unlock(&dev->mode_config.mutex);
> > -		return 0;
> > +		goto unlock;
> >  	}
> > +
> >  	DRM_DEBUG_KMS("\n");
> >  
> >  	drm_setup_crtcs(fb_helper, fb_helper->fb->width, fb_helper->fb->height);
> >  
> >  	mutex_unlock(&dev->mode_config.mutex);
> > +	mutex_unlock(&fb_helper->lock);
> >  
> >  	drm_fb_helper_set_par(fb_helper->fbdev);
> >  
> >  	return 0;
> > +
> > +unlock:
> > +	mutex_unlock(&fb_helper->lock);
> > +	return err;
> >  }
> >  EXPORT_SYMBOL(drm_fb_helper_hotplug_event);
> >  
> > diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
> > index 119e5e4609c7..33771d0c44e3 100644
> > --- a/include/drm/drm_fb_helper.h
> > +++ b/include/drm/drm_fb_helper.h
> > @@ -201,6 +201,13 @@ struct drm_fb_helper {
> >  	struct work_struct resume_work;
> >  
> >  	/**
> > +	 * @lock:
> > +	 *
> > +	 * Top-level FB helper lock.
> 
> Bit more detail would be good here.
> 
> "This protects all the internal data structures and lists, like
> @connector_info or @crtc_info."
> 
> > +	 */
> > +	struct mutex lock;
> > +
> > +	/**
> >  	 * @kernel_fb_list:
> >  	 *
> >  	 * Entry on the global kernel_fb_helper_list, used for kgdb entry/exit.
> 
> So I think you're missing to protect almost all the fbdev paths.
> Essentially anything looking at crtc_info or connector_info must hold the
> new lock, which means all the places that currently call
> drm_modeset_lock_all() need to grab this new lock too.
> 
> Best way to make sure you catch them all is to also update the lockdep
> assert in drm_fb_helper_for_each_connector().
> 
> There's still a massive mess going with locking here, especially around
> the probe code and drm_fb_helper_is_bound, but that's a different thing
> and better left for cleanup later on.
> -Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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