diff mbox series

[v4,1/3] drm: Use XArray instead of IDR for minors

Message ID 20220906201629.419160-2-michal.winiarski@intel.com (mailing list archive)
State New, archived
Headers show
Series drm: Use full allocated minor range for DRM | expand

Commit Message

Michał Winiarski Sept. 6, 2022, 8:16 p.m. UTC
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@intel.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
---
 drivers/gpu/drm/drm_drv.c | 49 ++++++++++++++-------------------------
 1 file changed, 17 insertions(+), 32 deletions(-)

Comments

Matthew Wilcox (Oracle) Sept. 6, 2022, 9:02 p.m. UTC | #1
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@intel.com>
> Suggested-by: Matthew Wilcox <willy@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.

> @@ -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!
Michał Winiarski Sept. 11, 2022, 9:13 p.m. UTC | #2
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@intel.com>
> > Suggested-by: Matthew Wilcox <willy@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
diff mbox series

Patch

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 8214a0b1ab7f..41799e4d0432 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 +98,18 @@  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);
 }
 
 static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
 {
 	struct drm_minor *minor;
-	unsigned long flags;
+	u32 id;
 	int r;
 
 	minor = drmm_kzalloc(dev, sizeof(*minor), GFP_KERNEL);
@@ -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;
 
 	r = drmm_add_action_or_reset(dev, drm_minor_alloc_release, minor);
 	if (r)
@@ -152,7 +141,7 @@  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 +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);
 
 	DRM_DEBUG("new minor registered %d\n", minor->index);
 	return 0;
@@ -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);
 
 	device_del(minor->kdev);
 	dev_set_drvdata(minor->kdev, NULL); /* safety belt */
@@ -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);
 
 	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);
+	xa_destroy(&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();