On Tue, Sep 06, 2022 at 10:16:27PM +0200, Michał Winiarski 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>
I have a few questions, but I like where you're going.
> @@ -98,21 +98,18 @@ 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_release(&drm_minors_xa, minor->index);
Has it definitely been unused at this point? I would think that
xa_erase() (an unconditional store) would be the correct function to
call.
> @@ -122,20 +119,12 @@ 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, &id, NULL,
> + XA_LIMIT(64 * type, 64 * (type + 1) - 1), GFP_KERNEL);
> if (r < 0)
> return r;
>
> - minor->index = r;
> + minor->index = id;
Wouldn't it be better to call:
r = xa_alloc(&drm_minors_xa, &minor->index, NULL,
XA_LIMIT(64 * type, 64 * (type + 1) - 1), GFP_KERNEL);
I might also prefer a little syntactic sugar like:
#define DRM_MINOR_LIMIT(type) XA_LIMIT(64 * (type), 64 * (type) + 63)
but that's definitely a matter of taste.
> @@ -172,9 +161,12 @@ 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_store(&drm_minors_xa, minor->index, &minor, GFP_KERNEL);
> + if (xa_is_err(entry)) {
> + ret = xa_err(entry);
> + goto err_debugfs;
> + }
> + WARN_ON(entry);
Might be better as an xa_cmpxchg()?
> @@ -187,16 +179,13 @@ 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_erase(&drm_minors_xa, minor->index);
This isn't an exact replacement, but I'm not sure whether that makes a
difference. xa_erase() allows allocation of this ID again while
idr_replace() means that lookups return NULL, but the ID remains in
use. The equivalent of idr_replace() is:
xa_store(&drm_minors_xa, minor->index, NULL, GFP_KERNEL);
> @@ -215,13 +204,10 @@ 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);
> + minor = xa_load(&drm_minors_xa, minor_id);
> if (minor)
> drm_dev_get(minor->dev);
> - spin_unlock_irqrestore(&drm_minor_lock, flags);
This is also not an exact equivalent as the dev_drm_get() is now outside
the lock. Does that matter in this case? I don't know the code well
enough to say. If you want it to be equivalent, then:
xa_lock(&drm_minors_xa);
minor = xa_load(&drm_minors_xa, minor_id);
if (minor)
drm_dev_get(minor->dev);
xa_unlock(&drm_minors_xa);
would be the code to use.
> @@ -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);
> + xa_destroy(&drm_minors_xa);
I don't know if this is the right call. xa_destroy() is the exact
replacement, but if the xarray should already be empty (and it should,
right?) then asserting the xa_empty() is true may be the better call
to make.
Phew, that was a lot of comments/questions. I hope that was useful!
