[PATCH 4/4] drm: Nerv drm_global_mutex BKL for good drivers

[Daniel Vetter]

On Tue, Jan 28, 2020 at 11:00:32AM +0000, Chris Wilson wrote:
> Quoting Daniel Vetter (2020-01-28 10:46:01)
> > This catches the majority of drivers (unfortunately not if we take
> > users into account, because all the big drivers have at least a
> > lastclose hook).
> 
> Yeah, that lastclose for switching back to fbcon, and ensuring that
> switch is started before the next client takes over the console, does
> nerf the ability of avoiding the global_mutex.
> 
> > 
> > With the prep patches out of the way all drm state is fully protected
> > and either prevents or can deal with the races from dropping the BKL
> > around open/close. The only thing left to audit are the various driver
> > hooks - by keeping the BKL around if any of them are set we have a
> > very simple cop-out!
> > 
> > Note that one of the biggest prep pieces to get here was making
> > dev->open_count atomic, which was done in
> > 
> > commit 7e13ad896484a0165a68197a2e64091ea28c9602
> > Author: Chris Wilson <chris at chris-wilson.co.uk>
> > Date:   Fri Jan 24 13:01:07 2020 +0000
> > 
> >     drm: Avoid drm_global_mutex for simple inc/dec of dev->open_count
> > 
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  drivers/gpu/drm/drm_drv.c      |  6 +++--
> >  drivers/gpu/drm/drm_file.c     | 46 ++++++++++++++++++++++++++++++----
> >  drivers/gpu/drm/drm_internal.h |  1 +
> >  3 files changed, 46 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> > index 05bdf0b9d2b3..9fcd6ab3c154 100644
> > --- a/drivers/gpu/drm/drm_drv.c
> > +++ b/drivers/gpu/drm/drm_drv.c
> > @@ -946,7 +946,8 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
> >         struct drm_driver *driver = dev->driver;
> >         int ret;
> >  
> > -       mutex_lock(&drm_global_mutex);
> > +       if (drm_dev_needs_global_mutex(dev))
> > +               mutex_lock(&drm_global_mutex);
> >  
> >         if (dev->driver->load) {
> >                 if (!drm_core_check_feature(dev, DRIVER_LEGACY))
> > @@ -992,7 +993,8 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
> >         drm_minor_unregister(dev, DRM_MINOR_PRIMARY);
> >         drm_minor_unregister(dev, DRM_MINOR_RENDER);
> >  out_unlock:
> > -       mutex_unlock(&drm_global_mutex);
> > +       if (drm_dev_needs_global_mutex(dev))
> > +               mutex_unlock(&drm_global_mutex);
> >         return ret;
> >  }
> >  EXPORT_SYMBOL(drm_dev_register);
> > diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> > index d36cb74ebe0c..efd6fe0b6b4f 100644
> > --- a/drivers/gpu/drm/drm_file.c
> > +++ b/drivers/gpu/drm/drm_file.c
> > @@ -51,6 +51,37 @@
> >  /* from BKL pushdown */
> >  DEFINE_MUTEX(drm_global_mutex);
> >  
> > +bool drm_dev_needs_global_mutex(struct drm_device *dev)
> > +{
> > +       /*
> > +        * Legacy drivers rely on all kinds of BKL locking semantics, don't
> > +        * bother. They also still need BKL locking for their ioctls, so better
> > +        * safe than sorry.
> > +        */
> > +       if (drm_core_check_feature(dev, DRIVER_LEGACY))
> > +               return true;
> > +
> > +       /*
> > +        * The deprecated ->load callback must be called after the driver is
> > +        * already registered. This means such drivers rely on the BKL to make
> > +        * sure an open can't proceed until the driver is actually fully set up.
> > +        * Similar hilarity holds for the unload callback.
> > +        */
> > +       if (dev->driver->load || dev->driver->unload)
> > +               return true;
> > +
> > +       /*
> > +        * Drivers with the lastclose callback assume that it's synchronized
> > +        * against concurrent opens, which again needs the BKL. The proper fix
> > +        * is to use the drm_client infrastructure with proper locking for each
> > +        * client.
> > +        */
> > +       if (dev->driver->lastclose)
> > +               return true;
> > +
> > +       return false;
> 
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
> I'm not particularly fussed by this patch, maybe a bit too obviously
> midlayer.

Yeah it's not great. But I think long-term we might have a chance to limit
this to DRIVER_LEGACY:
- load/unload is on the way out
- generic fbdev won't need lastclose
- I have an idea for fixing the vgaswitcheroo locking, now that I starred
  at this wall a bit more. Should be a good undependent cleanup.
- we could perhaps do a ->lastclose_unlocked for reducing that midlayer
  smell a tad. Same way we slowly managed to get rid of the
  dev->struct_mutex locking midlayer.

> I was wondering if we should have a *dev->global_mutex which drivers can
> set to be any level they need (with a bit of care on our part to make
> sure it is not destroyed across dev->driver->lastclose).

I feel like outside of legacy driver the drm_global_mutex protection is
limited enough that there's no need to give driver writers ideas :-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch