Re: [PATCH 5/5] drm: Nerv drm_global_mutex BKL for good drivers

From: Daniel Vetter <daniel@ffwll.ch>
To: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 5/5] drm: Nerv drm_global_mutex BKL for good drivers
Date: Wed, 29 Jan 2020 18:07:21 +0100	[thread overview]
Message-ID: <20200129170721.GN43062@phenom.ffwll.local> (raw)
In-Reply-To: <20200129164733.GB22331@ravnborg.org>

On Wed, Jan 29, 2020 at 05:47:33PM +0100, Sam Ravnborg wrote:
> Hi Daniel.
> 
> In the nit-pick department today - sorry.
> 
> Subject: [PATCH 5/5] drm: Nerv drm_global_mutex BKL for good drivers
> I did not understand this subject... - what is "Nerv"?

It's a typo, supposed to be nerf:

https://www.urbandictionary.com/define.php?term=nerf

Cheers, Daniel
> 
> 	Sam
> 
> On Wed, Jan 29, 2020 at 09:24:10AM +0100, Daniel Vetter wrote:
> > 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).
> > 
> > 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@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
> > 
> > Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Signed-off-by: Daniel Vetter <daniel.vetter@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;
> > +}
> > +
> >  /**
> >   * DOC: file operations
> >   *
> > @@ -378,9 +409,10 @@ int drm_open(struct inode *inode, struct file *filp)
> >  	if (IS_ERR(minor))
> >  		return PTR_ERR(minor);
> >  
> > -	mutex_unlock(&drm_global_mutex);
> > -
> >  	dev = minor->dev;
> > +	if (drm_dev_needs_global_mutex(dev))
> > +		mutex_unlock(&drm_global_mutex);
> > +
> >  	if (!atomic_fetch_inc(&dev->open_count))
> >  		need_setup = 1;
> >  
> > @@ -398,13 +430,15 @@ int drm_open(struct inode *inode, struct file *filp)
> >  		}
> >  	}
> >  
> > -	mutex_unlock(&drm_global_mutex);
> > +	if (drm_dev_needs_global_mutex(dev))
> > +		mutex_unlock(&drm_global_mutex);
> >  
> >  	return 0;
> >  
> >  err_undo:
> >  	atomic_dec(&dev->open_count);
> > -	mutex_unlock(&drm_global_mutex);
> > +	if (drm_dev_needs_global_mutex(dev))
> > +		mutex_unlock(&drm_global_mutex);
> >  	drm_minor_release(minor);
> >  	return retcode;
> >  }
> > @@ -444,6 +478,7 @@ int drm_release(struct inode *inode, struct file *filp)
> >  	struct drm_minor *minor = file_priv->minor;
> >  	struct drm_device *dev = minor->dev;
> >  
> > +	if (drm_dev_needs_global_mutex(dev))
> >  	mutex_lock(&drm_global_mutex);
> >  
> >  	DRM_DEBUG("open_count = %d\n", atomic_read(&dev->open_count));
> > @@ -453,7 +488,8 @@ int drm_release(struct inode *inode, struct file *filp)
> >  	if (atomic_dec_and_test(&dev->open_count))
> >  		drm_lastclose(dev);
> >  
> > -	mutex_unlock(&drm_global_mutex);
> > +	if (drm_dev_needs_global_mutex(dev))
> > +		mutex_unlock(&drm_global_mutex);
> >  
> >  	drm_minor_release(minor);
> >  
> > diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> > index 6937bf923f05..aeec2e68d772 100644
> > --- a/drivers/gpu/drm/drm_internal.h
> > +++ b/drivers/gpu/drm/drm_internal.h
> > @@ -41,6 +41,7 @@ struct drm_printer;
> >  
> >  /* drm_file.c */
> >  extern struct mutex drm_global_mutex;
> > +bool drm_dev_needs_global_mutex(struct drm_device *dev);
> >  struct drm_file *drm_file_alloc(struct drm_minor *minor);
> >  void drm_file_free(struct drm_file *file);
> >  void drm_lastclose(struct drm_device *dev);
> > -- 
> > 2.24.1
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel

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