Re: [Intel-gfx] [PATCH v6 1/4] drm: Use XArray instead of IDR for minors

[James Zhu]

On 2023-08-28 17:08, Michał Winiarski wrote:

On Fri, Aug 25, 2023 at 12:59:26PM -0400, James Zhu wrote:

On 2023-07-24 17:14, 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.winiar...@intel.com>
Suggested-by: Matthew Wilcox<wi...@infradead.org>
---
   drivers/gpu/drm/drm_drv.c | 63 ++++++++++++++++-----------------------
   1 file changed, 25 insertions(+), 38 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 3eda026ffac6..3faecb01186f 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_accel.h>
   #include <drm/drm_cache.h>
@@ -54,8 +55,7 @@ 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,
@@ -101,26 +101,23 @@ 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);
-       if (minor->type == DRM_MINOR_ACCEL) {
+       if (minor->type == DRM_MINOR_ACCEL)
                accel_minor_remove(minor->index);
-       } else {
-               spin_lock_irqsave(&drm_minor_lock, flags);
-               idr_remove(&drm_minors_idr, minor->index);
-               spin_unlock_irqrestore(&drm_minor_lock, flags);
-       }
+       else
+               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, enum drm_minor_type type)
   {
        struct drm_minor *minor;
-       unsigned long flags;
-       int r;
+       int index, r;
        minor = drmm_kzalloc(dev, sizeof(*minor), GFP_KERNEL);
        if (!minor)
@@ -129,24 +126,17 @@ static int drm_minor_alloc(struct drm_device *dev, enum 
drm_minor_type type)
        minor->type = type;
        minor->dev = dev;
-       idr_preload(GFP_KERNEL);
        if (type == DRM_MINOR_ACCEL) {
                r = accel_minor_alloc();
+               index = r;
        } else {
-               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);
+               r = xa_alloc(&drm_minors_xa, &index, NULL, 
DRM_MINOR_LIMIT(type), GFP_KERNEL);
        }
-       idr_preload_end();
        if (r < 0)
                return r;
-       minor->index = r;
+       minor->index = index;
        r = drmm_add_action_or_reset(dev, drm_minor_alloc_release, minor);
        if (r)
@@ -163,7 +153,7 @@ static int drm_minor_alloc(struct drm_device *dev, enum 
drm_minor_type type)
   static int drm_minor_register(struct drm_device *dev, enum drm_minor_type 
type)
   {
        struct drm_minor *minor;
-       unsigned long flags;
+       void *entry;
        int ret;
        DRM_DEBUG("\n");
@@ -190,9 +180,12 @@ static int drm_minor_register(struct drm_device *dev, enum 
drm_minor_type type)
        if (minor->type == DRM_MINOR_ACCEL) {
                accel_minor_replace(minor, minor->index);
        } else {
-               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);

[JZ] would WARN_ON(entry != minor)be better?

We expect NULL here.
The same one that was previously stored here ⌄⌄⌄
r = xa_alloc(&drm_minors_xa, &minor->index, NULL, DRM_EXTENDED_MINOR_LIMIT, 
GFP_KERNEL);
in drm_minor_alloc.

[JZ] My mistake.

        }
        DRM_DEBUG("new minor registered %d\n", minor->index);
@@ -206,20 +199,16 @@ static int drm_minor_register(struct drm_device *dev, 
enum drm_minor_type type)
   static void drm_minor_unregister(struct drm_device *dev, enum drm_minor_type 
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 */
-       if (minor->type == DRM_MINOR_ACCEL) {
+       if (minor->type == DRM_MINOR_ACCEL)
                accel_minor_replace(NULL, minor->index);
-       } else {
-               spin_lock_irqsave(&drm_minor_lock, flags);
-               idr_replace(&drm_minors_idr, NULL, minor->index);
-               spin_unlock_irqrestore(&drm_minor_lock, flags);
-       }
+       else
+               xa_store(&drm_minors_xa, minor->index, NULL, GFP_KERNEL);
        device_del(minor->kdev);
        dev_set_drvdata(minor->kdev, NULL); /* safety belt */
@@ -238,13 +227,12 @@ static void drm_minor_unregister(struct drm_device *dev, 
enum drm_minor_type typ
   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);

[JZ] why minor->dev need ca_lock here?

We're relying on the ordering for acquire/release (previously
guaranteed by the drm_minor_lock, now by internal XArray locking).
xa_load doesn't take xa_lock internally:
https://docs.kernel.org/core-api/xarray.html#locking

[JZ] I means that drm_dev_get(minor->dev); can move out of xa_lock.

-       spin_unlock_irqrestore(&drm_minor_lock, flags);
+       xa_unlock(&drm_minors_xa);
        if (!minor) {
                return ERR_PTR(-ENODEV);
@@ -1067,7 +1055,7 @@ static void drm_core_exit(void)
        unregister_chrdev(DRM_MAJOR, "drm");
        debugfs_remove(drm_debugfs_root);
        drm_sysfs_destroy();
-       idr_destroy(&drm_minors_idr);

[JZ] Should we call xa_destroy instead here?

We could, if we expect the xarray to potentially not be empty.
 From what I understand - all minors should be released at this point.

[JZ] In practice,  adding destroy here will be better.

Thanks,
-Michał

+       WARN_ON(!xa_empty(&drm_minors_xa));
        drm_connector_ida_destroy();
   }
@@ -1076,7 +1064,6 @@ 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();