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Message-ID: <20220911211353.kuv3jnwkgo2m5arw@nostramo>
Date: Sun, 11 Sep 2022 23:13:53 +0200
From: Michał Winiarski <michal.winiarski@...el.com>
To: Matthew Wilcox <willy@...radead.org>
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>
Subject: Re: [PATCH v4 1/3] drm: Use XArray instead of IDR for minors
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@...el.com>
> > Suggested-by: Matthew Wilcox <willy@...radead.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
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