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

[Sam Ravnborg]

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"?

	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 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
> 
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> 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;
> +}
> +
>  /**
>   * 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
> 
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