Re: [PATCH] drm: Use global_mutex to serialise open/lastclose

[Thomas Hellstrom <thomas@shipmail.org>]

On 08/08/2010 10:37 AM, Chris Wilson wrote:
> Dave Airlie:
>    I might be missing something, but what stops the race with something
>    reopening while we are in lastclose now?
>
> The global_mutex which appears to fill this role is only taken inside
> the drm_stub_open() and not drm_open() itself. As that mutex appears to
> serve no other role, retask it to serialise open/lastclose.
>    

Hmm.

But, if used this way, drm_global_mutex needs to protect *all* accesses
to dev::open_count, what is count_clock used for? Can't it be removed?

/Thomas


> Signed-off-by: Chris Wilson<chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/drm_fops.c |   35 ++++++++++++++++++-----------------
>   1 files changed, 18 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
> index 6367961..71bcd1b 100644
> --- a/drivers/gpu/drm/drm_fops.c
> +++ b/drivers/gpu/drm/drm_fops.c
> @@ -123,27 +123,30 @@ int drm_open(struct inode *inode, struct file *filp)
>   	struct drm_device *dev = NULL;
>   	int minor_id = iminor(inode);
>   	struct drm_minor *minor;
> -	int retcode = 0;
> +	int retcode = -ENODEV;
>
> +	mutex_lock(&drm_global_mutex);
>   	minor = idr_find(&drm_minors_idr, minor_id);
>   	if (!minor)
> -		return -ENODEV;
> +		goto err;
>
>   	if (!(dev = minor->dev))
> -		return -ENODEV;
> +		goto err;
>
>   	retcode = drm_open_helper(inode, filp, dev);
> -	if (!retcode) {
> -		atomic_inc(&dev->counts[_DRM_STAT_OPENS]);
> -		spin_lock(&dev->count_lock);
> -		if (!dev->open_count++) {
> -			spin_unlock(&dev->count_lock);
> -			retcode = drm_setup(dev);
> -			goto out;
> -		}
> +	if (retcode)
> +		goto err;
> +
> +	atomic_inc(&dev->counts[_DRM_STAT_OPENS]);
> +	spin_lock(&dev->count_lock);
> +	if (!dev->open_count++) {
>   		spin_unlock(&dev->count_lock);
> -	}
> -out:
> +		retcode = drm_setup(dev);
> +	} else
> +		spin_unlock(&dev->count_lock);
> +err:
> +	mutex_unlock(&drm_global_mutex);
> +
>   	if (!retcode) {
>   		mutex_lock(&dev->struct_mutex);
>   		if (minor->type == DRM_MINOR_LEGACY) {
> @@ -178,7 +181,6 @@ int drm_stub_open(struct inode *inode, struct file *filp)
>
>   	DRM_DEBUG("\n");
>
> -	mutex_lock(&drm_global_mutex);
>   	minor = idr_find(&drm_minors_idr, minor_id);
>   	if (!minor)
>   		goto out;
> @@ -198,8 +200,8 @@ int drm_stub_open(struct inode *inode, struct file *filp)
>   	}
>   	fops_put(old_fops);
>
> +	ret = 0;
>   out:
> -	mutex_unlock(&drm_global_mutex);
>   	return err;
>   }
>
> @@ -474,8 +476,6 @@ int drm_release(struct inode *inode, struct file *filp)
>   	struct drm_device *dev = file_priv->minor->dev;
>   	int retcode = 0;
>
> -	mutex_lock(&drm_global_mutex);
> -
>   	DRM_DEBUG("open_count = %d\n", dev->open_count);
>
>   	if (dev->driver->preclose)
> @@ -569,6 +569,7 @@ int drm_release(struct inode *inode, struct file *filp)
>   	 * End inline drm_release
>   	 */
>
> +	mutex_lock(&drm_global_mutex);
>   	atomic_inc(&dev->counts[_DRM_STAT_CLOSES]);
>   	spin_lock(&dev->count_lock);
>   	if (!--dev->open_count) {
>