[PATCH] drm/framebuffer: Acquire internal references on GEM handles
Christian König
christian.koenig at amd.com
Fri Jul 4 12:06:07 UTC 2025
On 04.07.25 10:53, Thomas Zimmermann wrote:
> Acquire GEM handles in drm_framebuffer_init() and release them in
> the corresponding drm_framebuffer_cleanup(). Ties the handle's
> lifetime to the framebuffer. Not all GEM buffer objects have GEM
> handles. If not set, no refcounting takes place. This is the case
> for some fbdev emulation. This is not a problem as these GEM objects
> do not use dma-bufs and drivers will not release them while fbdev
> emulation is running.
>
> As all drivers use drm_framebuffer_init(), they will now all hold
> dma-buf references as fixed in commit 5307dce878d4 ("drm/gem: Acquire
> references on GEM handles for framebuffers").
>
> In the GEM framebuffer helpers, restore the original ref counting
> on buffer objects. As the helpers for handle refcounting are now
> no longer called from outside the DRM core, unexport the symbols.
>
> Gma500 (unnecessarily) clears the framebuffer's GEM-object pointer
> before calling drm_framebuffer_cleanup(). Remove these lines to
> make it consistent with the rest of the drivers. It's one of the
> fbdev emulations with no GEM handle on their buffers. The change
> to gma500 is therefore rather cosmetic.
Could we separate that change out? I mean we want to backport the patch.
>
> Tested on i915, amdgpu (by Bert) and gma500. Also tested on i915
> plus udl for the original problem with dma-buf sharing.
>
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> Fixes: 5307dce878d4 ("drm/gem: Acquire references on GEM handles for framebuffers")
> Reported-by: Bert Karwatzki <spasswolf at web.de>
> Closes: https://lore.kernel.org/dri-devel/20250703115915.3096-1-spasswolf@web.de/
> Tested-by: Bert Karwatzki <spasswolf at web.de>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: Anusha Srivatsa <asrivats at redhat.com>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Maxime Ripard <mripard at kernel.org>
> Cc: Sumit Semwal <sumit.semwal at linaro.org>
> Cc: "Christian König" <christian.koenig at amd.com>
> Cc: linux-media at vger.kernel.org
> Cc: dri-devel at lists.freedesktop.org
> Cc: linaro-mm-sig at lists.linaro.org
> Cc: <stable at vger.kernel.org>
> ---
> drivers/gpu/drm/drm_framebuffer.c | 23 +++++++-
> drivers/gpu/drm/drm_gem.c | 59 +++++++++++++-------
> drivers/gpu/drm/drm_gem_framebuffer_helper.c | 16 +++---
> drivers/gpu/drm/drm_internal.h | 4 +-
> drivers/gpu/drm/gma500/fbdev.c | 2 -
> 5 files changed, 69 insertions(+), 35 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> index b781601946db..e4a10dd053fc 100644
> --- a/drivers/gpu/drm/drm_framebuffer.c
> +++ b/drivers/gpu/drm/drm_framebuffer.c
> @@ -862,11 +862,17 @@ EXPORT_SYMBOL_FOR_TESTS_ONLY(drm_framebuffer_free);
> int drm_framebuffer_init(struct drm_device *dev, struct drm_framebuffer *fb,
> const struct drm_framebuffer_funcs *funcs)
> {
> + unsigned int i;
> int ret;
>
> if (WARN_ON_ONCE(fb->dev != dev || !fb->format))
> return -EINVAL;
>
> + for (i = 0; i < fb->format->num_planes; i++) {
> + if (fb->obj[i])
> + drm_gem_object_handle_get_if_exists_unlocked(fb->obj[i]);
> + }
> +
> INIT_LIST_HEAD(&fb->filp_head);
>
> fb->funcs = funcs;
> @@ -875,7 +881,7 @@ int drm_framebuffer_init(struct drm_device *dev, struct drm_framebuffer *fb,
> ret = __drm_mode_object_add(dev, &fb->base, DRM_MODE_OBJECT_FB,
> false, drm_framebuffer_free);
> if (ret)
> - goto out;
> + goto err;
>
> mutex_lock(&dev->mode_config.fb_lock);
> dev->mode_config.num_fb++;
> @@ -883,7 +889,14 @@ int drm_framebuffer_init(struct drm_device *dev, struct drm_framebuffer *fb,
> mutex_unlock(&dev->mode_config.fb_lock);
>
> drm_mode_object_register(dev, &fb->base);
> -out:
> +
> + return 0;
> +
> +err:
> + for (i = 0; i < fb->format->num_planes; i++) {
> + if (fb->obj[i])
> + drm_gem_object_handle_put_if_exists_unlocked(fb->obj[i]);
> + }
> return ret;
> }
> EXPORT_SYMBOL(drm_framebuffer_init);
> @@ -960,6 +973,12 @@ EXPORT_SYMBOL(drm_framebuffer_unregister_private);
> void drm_framebuffer_cleanup(struct drm_framebuffer *fb)
> {
> struct drm_device *dev = fb->dev;
> + unsigned int i;
> +
> + for (i = 0; i < fb->format->num_planes; i++) {
> + if (fb->obj[i])
> + drm_gem_object_handle_put_if_exists_unlocked(fb->obj[i]);
> + }
That goes boom as soon as somebody grabs a GEM handle for the FB used for fbdev emulation (which is perfectly possible with the UAPI but not done currently as far as I know).
It's probably ok for a bug fix we are going to backport, but a more robust long term solution is really desired here I think.
Regards,
Christian.
>
> mutex_lock(&dev->mode_config.fb_lock);
> list_del(&fb->head);
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index bc505d938b3e..9d8b9e6b7d25 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -224,23 +224,27 @@ static void drm_gem_object_handle_get(struct drm_gem_object *obj)
> }
>
> /**
> - * drm_gem_object_handle_get_unlocked - acquire reference on user-space handles
> + * drm_gem_object_handle_get_if_exists_unlocked - acquire reference on user-space handle, if any
> * @obj: GEM object
> *
> - * Acquires a reference on the GEM buffer object's handle. Required
> - * to keep the GEM object alive. Call drm_gem_object_handle_put_unlocked()
> - * to release the reference.
> + * Acquires a reference on the GEM buffer object's handle. Required to keep
> + * the GEM object alive. Call drm_gem_object_handle_put_if_exists_unlocked()
> + * to release the reference. Does nothing if the buffer object has no handle.
> */
> -void drm_gem_object_handle_get_unlocked(struct drm_gem_object *obj)
> +void drm_gem_object_handle_get_if_exists_unlocked(struct drm_gem_object *obj)
> {
> struct drm_device *dev = obj->dev;
>
> guard(mutex)(&dev->object_name_lock);
>
> - drm_WARN_ON(dev, !obj->handle_count); /* first ref taken in create-tail helper */
> - drm_gem_object_handle_get(obj);
> + /*
> + * First ref taken during GEM object creation, if any. Some
> + * drivers set up internal framebuffers with GEM objects that
> + * do not have a GEM handle. Hence, this counter can be zero.
> + */
> + if (obj->handle_count)
> + drm_gem_object_handle_get(obj);
> }
> -EXPORT_SYMBOL(drm_gem_object_handle_get_unlocked);
>
> /**
> * drm_gem_object_handle_free - release resources bound to userspace handles
> @@ -272,21 +276,11 @@ static void drm_gem_object_exported_dma_buf_free(struct drm_gem_object *obj)
> }
> }
>
> -/**
> - * drm_gem_object_handle_put_unlocked - releases reference on user-space handles
> - * @obj: GEM object
> - *
> - * Releases a reference on the GEM buffer object's handle. Possibly releases
> - * the GEM buffer object and associated dma-buf objects.
> - */
> -void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
> +static void drm_gem_object_handle_put_unlocked_tail(struct drm_gem_object *obj)
> {
> struct drm_device *dev = obj->dev;
> bool final = false;
>
> - if (WARN_ON(READ_ONCE(obj->handle_count) == 0))
> - return;
> -
> /*
> * Must bump handle count first as this may be the last
> * ref, in which case the object would disappear before we
> @@ -304,7 +298,32 @@ void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
> if (final)
> drm_gem_object_put(obj);
> }
> -EXPORT_SYMBOL(drm_gem_object_handle_put_unlocked);
> +
> +static void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj)
> +{
> + struct drm_device *dev = obj->dev;
> +
> + if (drm_WARN_ON(dev, READ_ONCE(obj->handle_count) == 0))
> + return;
> +
> + drm_gem_object_handle_put_unlocked_tail(obj);
> +}
> +
> +/**
> + * drm_gem_object_handle_put_if_exists_unlocked - releases reference on user-space handle, if any
> + * @obj: GEM object
> + *
> + * Releases a reference on the GEM buffer object's handle. Possibly releases
> + * the GEM buffer object and associated dma-buf objects. Does nothing if the
> + * buffer object has no handle.
> + */
> +void drm_gem_object_handle_put_if_exists_unlocked(struct drm_gem_object *obj)
> +{
> + if (!obj->handle_count)
> + return;
> +
> + drm_gem_object_handle_put_unlocked_tail(obj);
> +}
>
> /*
> * Called at device or object close to release the file's
> diff --git a/drivers/gpu/drm/drm_gem_framebuffer_helper.c b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> index c60d0044d036..618ce725cd75 100644
> --- a/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> +++ b/drivers/gpu/drm/drm_gem_framebuffer_helper.c
> @@ -100,7 +100,7 @@ void drm_gem_fb_destroy(struct drm_framebuffer *fb)
> unsigned int i;
>
> for (i = 0; i < fb->format->num_planes; i++)
> - drm_gem_object_handle_put_unlocked(fb->obj[i]);
> + drm_gem_object_put(fb->obj[i]);
>
> drm_framebuffer_cleanup(fb);
> kfree(fb);
> @@ -183,10 +183,8 @@ int drm_gem_fb_init_with_funcs(struct drm_device *dev,
> if (!objs[i]) {
> drm_dbg_kms(dev, "Failed to lookup GEM object\n");
> ret = -ENOENT;
> - goto err_gem_object_handle_put_unlocked;
> + goto err_gem_object_put;
> }
> - drm_gem_object_handle_get_unlocked(objs[i]);
> - drm_gem_object_put(objs[i]);
>
> min_size = (height - 1) * mode_cmd->pitches[i]
> + drm_format_info_min_pitch(info, i, width)
> @@ -196,22 +194,22 @@ int drm_gem_fb_init_with_funcs(struct drm_device *dev,
> drm_dbg_kms(dev,
> "GEM object size (%zu) smaller than minimum size (%u) for plane %d\n",
> objs[i]->size, min_size, i);
> - drm_gem_object_handle_put_unlocked(objs[i]);
> + drm_gem_object_put(objs[i]);
> ret = -EINVAL;
> - goto err_gem_object_handle_put_unlocked;
> + goto err_gem_object_put;
> }
> }
>
> ret = drm_gem_fb_init(dev, fb, mode_cmd, objs, i, funcs);
> if (ret)
> - goto err_gem_object_handle_put_unlocked;
> + goto err_gem_object_put;
>
> return 0;
>
> -err_gem_object_handle_put_unlocked:
> +err_gem_object_put:
> while (i > 0) {
> --i;
> - drm_gem_object_handle_put_unlocked(objs[i]);
> + drm_gem_object_put(objs[i]);
> }
> return ret;
> }
> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> index f7b414a813ae..9233019f54a8 100644
> --- a/drivers/gpu/drm/drm_internal.h
> +++ b/drivers/gpu/drm/drm_internal.h
> @@ -161,8 +161,8 @@ void drm_sysfs_lease_event(struct drm_device *dev);
>
> /* drm_gem.c */
> int drm_gem_init(struct drm_device *dev);
> -void drm_gem_object_handle_get_unlocked(struct drm_gem_object *obj);
> -void drm_gem_object_handle_put_unlocked(struct drm_gem_object *obj);
> +void drm_gem_object_handle_get_if_exists_unlocked(struct drm_gem_object *obj);
> +void drm_gem_object_handle_put_if_exists_unlocked(struct drm_gem_object *obj);
> int drm_gem_handle_create_tail(struct drm_file *file_priv,
> struct drm_gem_object *obj,
> u32 *handlep);
> diff --git a/drivers/gpu/drm/gma500/fbdev.c b/drivers/gpu/drm/gma500/fbdev.c
> index 8edefea2ef59..afd252108cfa 100644
> --- a/drivers/gpu/drm/gma500/fbdev.c
> +++ b/drivers/gpu/drm/gma500/fbdev.c
> @@ -121,7 +121,6 @@ static void psb_fbdev_fb_destroy(struct fb_info *info)
> drm_fb_helper_fini(fb_helper);
>
> drm_framebuffer_unregister_private(fb);
> - fb->obj[0] = NULL;
> drm_framebuffer_cleanup(fb);
> kfree(fb);
>
> @@ -243,7 +242,6 @@ int psb_fbdev_driver_fbdev_probe(struct drm_fb_helper *fb_helper,
>
> err_drm_framebuffer_unregister_private:
> drm_framebuffer_unregister_private(fb);
> - fb->obj[0] = NULL;
> drm_framebuffer_cleanup(fb);
> kfree(fb);
> err_drm_gem_object_put:
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