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Message-ID: <bcbfe6d5-da87-fa2b-c05a-8bea6e0004fb@amd.com>
Date: Tue, 29 Aug 2023 13:34:22 -0400
From: James Zhu <jamesz@....com>
To: Michał Winiarski <michal.winiarski@...el.com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
intel-gfx@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
Daniel Vetter <daniel@...ll.ch>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>,
Simon Ser <contact@...rsion.fr>,
Matthew Wilcox <willy@...radead.org>,
Oded Gabbay <ogabbay@...nel.org>,
Christian König <christian.koenig@....com>,
James Zhu <James.Zhu@....com>,
Pekka Paalanen <pekka.paalanen@...labora.com>,
Emil Velikov <emil.l.velikov@...il.com>
Subject: Re: [PATCH v6 1/4] drm: Use XArray instead of IDR for minors
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.winiarski@...el.com>
>>> Suggested-by: Matthew Wilcox<willy@...radead.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();
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