[Intel-gfx] [PATCH v4 1/3] drm: Use XArray instead of IDR for minors

[Michał Winiarski]

On Tue, Sep 06, 2022 at 10:02:24PM +0100, Matthew Wilcox wrote:
> 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 at intel.com>
> > Suggested-by: Matthew Wilcox <willy at infradead.org>
> 
> 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.

Yes, unless there's a programmers error somewhere - I'll replace it though.

> 
> > @@ -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.

Sure.

> 
> > @@ -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()?

Ack.

> 
> > @@ -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);

It does makes a difference, I'll change it to the equivalent.

> 
> > @@ -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.

Again, it does matter, I'll change it.

> 
> > @@ -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.

Yeah - I'll replace it with WARN_ON.

> 
> 
> Phew, that was a lot of comments/questions.  I hope that was useful!

Very useful indeed, thanks!

-Michał