Re: [PATCH v5 1/3] drm: Use XArray instead of IDR for minors

[Oded Gabbay]

On Mon, Sep 12, 2022 at 12:17 AM Michał Winiarski
<michal.winiarski@xxxxxxxxx> wrote:
>
> IDR is deprecated, and since XArray manages its own state with internal
> locking, it simplifies the locking on DRM side.
> Additionally, don't use the IRQ-safe variant, since operating on drm
> minor is not done in IRQ context.
>
> Signed-off-by: Michał Winiarski <michal.winiarski@xxxxxxxxx>
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_drv.c | 51 ++++++++++++++-------------------------
>  1 file changed, 18 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 8214a0b1ab7f..61d24cdcd0f8 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -34,6 +34,7 @@
>  #include <linux/pseudo_fs.h>
>  #include <linux/slab.h>
>  #include <linux/srcu.h>
> +#include <linux/xarray.h>
>
>  #include <drm/drm_cache.h>
>  #include <drm/drm_client.h>
> @@ -53,8 +54,7 @@ MODULE_AUTHOR("Gareth Hughes, Leif Delgass, José Fonseca, Jon Smirl");
>  MODULE_DESCRIPTION("DRM shared core routines");
>  MODULE_LICENSE("GPL and additional rights");
>
> -static DEFINE_SPINLOCK(drm_minor_lock);
> -static struct idr drm_minors_idr;
> +static DEFINE_XARRAY_ALLOC(drm_minors_xa);
>
>  /*
>   * If the drm core fails to init for whatever reason,
> @@ -98,21 +98,19 @@ static struct drm_minor **drm_minor_get_slot(struct drm_device *dev,
>  static void drm_minor_alloc_release(struct drm_device *dev, void *data)
>  {
>         struct drm_minor *minor = data;
> -       unsigned long flags;
>
>         WARN_ON(dev != minor->dev);
>
>         put_device(minor->kdev);
>
> -       spin_lock_irqsave(&drm_minor_lock, flags);
> -       idr_remove(&drm_minors_idr, minor->index);
> -       spin_unlock_irqrestore(&drm_minor_lock, flags);
> +       xa_erase(&drm_minors_xa, minor->index);
>  }
>
> +#define DRM_MINOR_LIMIT(t) ({ typeof(t) _t = (t); XA_LIMIT(64 * _t, 64 * _t + 63); })
> +
>  static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
>  {
>         struct drm_minor *minor;
> -       unsigned long flags;
>         int r;
>
>         minor = drmm_kzalloc(dev, sizeof(*minor), GFP_KERNEL);
> @@ -122,21 +120,10 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
>         minor->type = type;
>         minor->dev = dev;
>
> -       idr_preload(GFP_KERNEL);
> -       spin_lock_irqsave(&drm_minor_lock, flags);
> -       r = idr_alloc(&drm_minors_idr,
> -                     NULL,
> -                     64 * type,
> -                     64 * (type + 1),
> -                     GFP_NOWAIT);
> -       spin_unlock_irqrestore(&drm_minor_lock, flags);
> -       idr_preload_end();
> -
> +       r = xa_alloc(&drm_minors_xa, &minor->index, NULL, DRM_MINOR_LIMIT(type), GFP_KERNEL);
>         if (r < 0)
>                 return r;
>
> -       minor->index = r;
> -
>         r = drmm_add_action_or_reset(dev, drm_minor_alloc_release, minor);
>         if (r)
>                 return r;
> @@ -152,7 +139,7 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
>  static int drm_minor_register(struct drm_device *dev, unsigned int type)
>  {
>         struct drm_minor *minor;
> -       unsigned long flags;
> +       void *entry;
>         int ret;
>
>         DRM_DEBUG("\n");
> @@ -172,9 +159,12 @@ static int drm_minor_register(struct drm_device *dev, unsigned int type)
>                 goto err_debugfs;
>
>         /* replace NULL with @minor so lookups will succeed from now on */
> -       spin_lock_irqsave(&drm_minor_lock, flags);
> -       idr_replace(&drm_minors_idr, minor, minor->index);
> -       spin_unlock_irqrestore(&drm_minor_lock, flags);
> +       entry = xa_cmpxchg(&drm_minors_xa, minor->index, NULL, &minor, GFP_KERNEL);
I believe we should pass in "minor", without the &, as &minor will
give you the address of the local pointer.

Oded

> +       if (xa_is_err(entry)) {
> +               ret = xa_err(entry);
> +               goto err_debugfs;
> +       }
> +       WARN_ON(entry);
>
>         DRM_DEBUG("new minor registered %d\n", minor->index);
>         return 0;
> @@ -187,16 +177,13 @@ static int drm_minor_register(struct drm_device *dev, unsigned int type)
>  static void drm_minor_unregister(struct drm_device *dev, unsigned int type)
>  {
>         struct drm_minor *minor;
> -       unsigned long flags;
>
>         minor = *drm_minor_get_slot(dev, type);
>         if (!minor || !device_is_registered(minor->kdev))
>                 return;
>
>         /* replace @minor with NULL so lookups will fail from now on */
> -       spin_lock_irqsave(&drm_minor_lock, flags);
> -       idr_replace(&drm_minors_idr, NULL, minor->index);
> -       spin_unlock_irqrestore(&drm_minor_lock, flags);
> +       xa_store(&drm_minors_xa, minor->index, NULL, GFP_KERNEL);
>
>         device_del(minor->kdev);
>         dev_set_drvdata(minor->kdev, NULL); /* safety belt */
> @@ -215,13 +202,12 @@ static void drm_minor_unregister(struct drm_device *dev, unsigned int type)
>  struct drm_minor *drm_minor_acquire(unsigned int minor_id)
>  {
>         struct drm_minor *minor;
> -       unsigned long flags;
>
> -       spin_lock_irqsave(&drm_minor_lock, flags);
> -       minor = idr_find(&drm_minors_idr, minor_id);
> +       xa_lock(&drm_minors_xa);
> +       minor = xa_load(&drm_minors_xa, minor_id);
>         if (minor)
>                 drm_dev_get(minor->dev);
> -       spin_unlock_irqrestore(&drm_minor_lock, flags);
> +       xa_unlock(&drm_minors_xa);
>
>         if (!minor) {
>                 return ERR_PTR(-ENODEV);
> @@ -1037,7 +1023,7 @@ static void drm_core_exit(void)
>         unregister_chrdev(DRM_MAJOR, "drm");
>         debugfs_remove(drm_debugfs_root);
>         drm_sysfs_destroy();
> -       idr_destroy(&drm_minors_idr);
> +       WARN_ON(!xa_empty(&drm_minors_xa));
>         drm_connector_ida_destroy();
>  }
>
> @@ -1046,7 +1032,6 @@ static int __init drm_core_init(void)
>         int ret;
>
>         drm_connector_ida_init();
> -       idr_init(&drm_minors_idr);
>         drm_memcpy_init_early();
>
>         ret = drm_sysfs_init();
> --
> 2.37.3
>