[PATCH 3/4] drm: Push drm_global_mutex locking in drm_open

[Chris Wilson]

Quoting Daniel Vetter (2020-01-28 10:46:00)
> We want to only take the BKL on crap drivers, but to know whether
> we have a crap driver we first need to look it up. Split this shuffle
> out from the main BKL-disabling patch, for more clarity.
> 
> Since the minors are refcounted drm_minor_acquire is purely internal
> and this does not have a driver visible effect.
> 
> v2: Push the locking even further into drm_open(), suggested by Chris.
> This gives us more symmetry with drm_release(), and maybe a futuer
> avenue where we make drm_globale_mutex locking (partially) opt-in like
> with drm_release_noglobal().
> 
> 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  | 14 +++++---------
>  drivers/gpu/drm/drm_file.c |  6 ++++++
>  2 files changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 8deff75b484c..05bdf0b9d2b3 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -1085,17 +1085,14 @@ static int drm_stub_open(struct inode *inode, struct file *filp)
>  
>         DRM_DEBUG("\n");
>  
> -       mutex_lock(&drm_global_mutex);
>         minor = drm_minor_acquire(iminor(inode));
> -       if (IS_ERR(minor)) {
> -               err = PTR_ERR(minor);
> -               goto out_unlock;
> -       }
> +       if (IS_ERR(minor))
> +               return PTR_ERR(minor);
>  
>         new_fops = fops_get(minor->dev->driver->fops);
>         if (!new_fops) {
>                 err = -ENODEV;
> -               goto out_release;
> +               goto out;
>         }
>  
>         replace_fops(filp, new_fops);
> @@ -1104,10 +1101,9 @@ static int drm_stub_open(struct inode *inode, struct file *filp)
>         else
>                 err = 0;
>  
> -out_release:
> +out:
>         drm_minor_release(minor);
> -out_unlock:
> -       mutex_unlock(&drm_global_mutex);
> +
>         return err;
>  }
>  
> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> index 1075b3a8b5b1..d36cb74ebe0c 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -378,6 +378,8 @@ 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 (!atomic_fetch_inc(&dev->open_count))
>                 need_setup = 1;
> @@ -395,10 +397,14 @@ int drm_open(struct inode *inode, struct file *filp)
>                         goto err_undo;
>                 }
>         }
> +
> +       mutex_unlock(&drm_global_mutex);

The only reason why I could think it was in drm_stub_open() not
drm_open() was for the possibility of some driver using a different
callback. Such a driver would not be partaking in the drm_global_mutex
so...
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris