[PATCH 1/2] drm: Release filp before global lock

[Daniel Vetter]

On Fri, Jan 24, 2020 at 12:56:26PM +0000, Chris Wilson wrote:
> The file is not part of the global drm resource and can be released
> prior to take the global mutex to drop the open_count (and potentially
> close) the drm device. As the global mutex is indeed global, not only
> within the device but across devices, a slow file release mechanism can
> bottleneck the entire system.
> 
> However, inside drm_close_helper() there are a number of dev->driver
> callbacks that take the drm_device as the first parameter... Worryingly
> some of those callbacks may be (implicitly) depending on the global
> mutex.
> 
> v2: Drop the debug message for the open-count, it's included with the
> drm_file_free() debug message -- and for good measure make that up as
> reading outside of the mutex.
> 
> v3: Separate the calling of the filp cleanup outside of
> drm_global_mutex into a new drm_release_noglobal() hook, so that we can
> phase the transition. drm/savage relies on the global mutex, and there
> may be more, so be cautious.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Thomas Hellström (VMware) <thomas_os at shipmail.org>
> Acked-by: Thomas Hellström (VMware) <thomas_os at shipmail.org>
> ---
>  drivers/gpu/drm/drm_file.c      | 36 ++++++++++++++++++++++++++++++++-
>  drivers/gpu/drm/i915/i915_drv.c |  2 +-
>  include/drm/drm_file.h          |  1 +
>  3 files changed, 37 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> index 92d16724f949..e25306c49cc6 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -220,7 +220,7 @@ void drm_file_free(struct drm_file *file)
>  	DRM_DEBUG("pid = %d, device = 0x%lx, open_count = %d\n",
>  		  task_pid_nr(current),
>  		  (long)old_encode_dev(file->minor->kdev->devt),
> -		  dev->open_count);
> +		  READ_ONCE(dev->open_count));
>  
>  	if (drm_core_check_feature(dev, DRIVER_LEGACY) &&
>  	    dev->driver->preclose)
> @@ -455,6 +455,40 @@ int drm_release(struct inode *inode, struct file *filp)
>  }
>  EXPORT_SYMBOL(drm_release);
>  
> +/**
> + * drm_release_noglobal - release method for DRM file
> + * @inode: device inode
> + * @filp: file pointer.
> + *
> + * This function may be used by drivers as their &file_operations.release
> + * method. It frees any resources associated with the open file prior to taking
> + * the drm_global_mutex, which then calls the &drm_driver.postclose driver
> + * callback. If this is the last open file for the DRM device also proceeds to
> + * call the &drm_driver.lastclose driver callback.
> + *
> + * RETURNS:
> + *
> + * Always succeeds and returns 0.
> + */
> +int drm_release_noglobal(struct inode *inode, struct file *filp)
> +{
> +	struct drm_file *file_priv = filp->private_data;
> +	struct drm_minor *minor = file_priv->minor;
> +	struct drm_device *dev = minor->dev;
> +
> +	drm_close_helper(filp);
> +
> +	mutex_lock(&drm_global_mutex);
> +	if (!--dev->open_count)
> +		drm_lastclose(dev);
> +	mutex_unlock(&drm_global_mutex);

btw my rough idea for lastclose is that we're just going to make it racy,
and then use the master lock and drm_client infrastructure to handle
fights between fbcon and a restarted compositor. That's already how that's
handled everywhere else. We might need to cut over drivers to the generic
fbcon stuff (or at least steal parts of the drm_client stuff I think), but
not sure.
-Daniel

> +
> +	drm_minor_release(minor);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_release_noglobal);
> +
>  /**
>   * drm_read - read method for DRM file
>   * @filp: file pointer
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index e9b42e962032..5a5846d892f4 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -2673,7 +2673,7 @@ const struct dev_pm_ops i915_pm_ops = {
>  static const struct file_operations i915_driver_fops = {
>  	.owner = THIS_MODULE,
>  	.open = drm_open,
> -	.release = drm_release,
> +	.release = drm_release_noglobal,
>  	.unlocked_ioctl = drm_ioctl,
>  	.mmap = i915_gem_mmap,
>  	.poll = drm_poll,
> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
> index 8b099b347817..19df8028a6c4 100644
> --- a/include/drm/drm_file.h
> +++ b/include/drm/drm_file.h
> @@ -374,6 +374,7 @@ int drm_open(struct inode *inode, struct file *filp);
>  ssize_t drm_read(struct file *filp, char __user *buffer,
>  		 size_t count, loff_t *offset);
>  int drm_release(struct inode *inode, struct file *filp);
> +int drm_release_noglobal(struct inode *inode, struct file *filp);
>  __poll_t drm_poll(struct file *filp, struct poll_table_struct *wait);
>  int drm_event_reserve_init_locked(struct drm_device *dev,
>  				  struct drm_file *file_priv,
> -- 
> 2.25.0
> 

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