On Sun, Nov 06, 2022 at 04:51:39PM +0200, Oded Gabbay wrote:
> On Wed, Nov 2, 2022 at 4:23 PM Oded Gabbay <ogabbay@xxxxxxxxxx> wrote:
> >
> > On Mon, Sep 12, 2022 at 12:17 AM Michał Winiarski
> > <michal.winiarski@xxxxxxxxx> 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>
> > > ---
> > > drivers/gpu/drm/drm_drv.c | 51 ++++++++++++++-------------------------
> > > 1 file changed, 18 insertions(+), 33 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> > > index 8214a0b1ab7f..61d24cdcd0f8 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 +98,19 @@ 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_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, unsigned int type)
> > > {
> > > struct drm_minor *minor;
> > > - unsigned long flags;
> > > int r;
> > >
> > > minor = drmm_kzalloc(dev, sizeof(*minor), GFP_KERNEL);
> > > @@ -122,21 +120,10 @@ 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, &minor->index, NULL, DRM_MINOR_LIMIT(type), GFP_KERNEL);
> This was GFP_NOWAIT in the original code.
Because we were using spinlock and idr_preload.
We're actually fine with GFP_KERNEL (and we could just use mutex with
idr):
https://lore.kernel.org/dri-devel/20220823210612.296922-3-michal.winiarski@xxxxxxxxx/
>
> > > if (r < 0)
> > > return r;
> > >
> > > - minor->index = r;
> > > -
> > > r = drmm_add_action_or_reset(dev, drm_minor_alloc_release, minor);
> > > if (r)
> > > return r;
> > > @@ -152,7 +139,7 @@ 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 +159,12 @@ 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_cmpxchg(&drm_minors_xa, minor->index, NULL, &minor, GFP_KERNEL);
> > I believe we should pass in "minor", without the &, as &minor will
> > give you the address of the local pointer.
> >
> > Oded
> >
> > > + 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 +177,13 @@ 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_store(&drm_minors_xa, minor->index, NULL, GFP_KERNEL);
> > >
> > > device_del(minor->kdev);
> > > dev_set_drvdata(minor->kdev, NULL); /* safety belt */
> > > @@ -215,13 +202,12 @@ 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);
> > > + xa_lock(&drm_minors_xa);
> > > + minor = xa_load(&drm_minors_xa, minor_id);
> Did you check this part ? Because this always gives me NULL...
>
> I tried executing the following code in a dummy driver I wrote:
>
> static DEFINE_XARRAY_ALLOC(xa_dummy);
> void check_xa(void *pdev)
> {
> void *entry;
> int ret, index;
>
> ret = xa_alloc(&xa_dummy, &index, NULL, XA_LIMIT(0, 63), GFP_NOWAIT);
> if (ret < 0)
> return ret;
>
> entry = xa_cmpxchg(&xa_dummy, index, NULL, pdev, GFP_KERNEL);
> if (xa_is_err(entry))
> return;
>
> xa_lock(&xa_dummy);
> xa_dev = xa_load(&xa_dummy, index);
> xa_unlock(&xa_dummy);
> }
>
> And to my surprise xa_dev is always NULL, when it should be pdev.
> I believe that because we first allocate the entry with NULL, it is
> considered a "zero" entry in the XA.
> And when we replace it, this attribute doesn't change so when we do
> xa_load, the xa code thinks the entry is a "zero" entry and returns
> NULL.
> If that's correct, you need to either fix xarray code or change the
> flow of allocating this in drm.
>
> If I send a real pointer (just a dummy object I allocated) instead of
> NULL in xa_alloc, and then do xa_cmpxchg with pdev, xa_load returns
> pdev successfully.
> That points to the NULL being problematic in allocating an entry.
Yeah, this was just using xa_store in previous revision.
https://lore.kernel.org/dri-devel/20220906201629.419160-2-michal.winiarski@xxxxxxxxx/
And I didn't notice the CI failures:
https://patchwork.freedesktop.org/series/108206/
since I was originally hoping for comments on extending DRM minor limit,
XArray conversion was a bit of an extra.
Sorry about that - I just sent a next version using xa_store as a
separate patch (without extending DRM device limit).
Thanks!
-Michał
>
> Oded
>
>
> > > if (minor)
> > > drm_dev_get(minor->dev);
> > > - spin_unlock_irqrestore(&drm_minor_lock, flags);
> > > + xa_unlock(&drm_minors_xa);
> > >
> > > 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);
> > > + WARN_ON(!xa_empty(&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();
> > > --
> > > 2.37.3
> > >
