Re: [PATCH] drm/core: Do not preserve framebuffer on rmfb, v4.

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 04/05/16 13:38, Maarten Lankhorst wrote:
> It turns out that preserving framebuffers after the rmfb call breaks
> vmwgfx userspace. This was originally introduced because it was thought
> nobody relied on the behavior, but unfortunately it seems there are
> exceptions.
>
> drm_framebuffer_remove may fail with -EINTR now, so a straight revert
> is impossible. There is no way to remove the framebuffer from the lists
> and active planes without introducing a race because of the different
> locking requirements. Instead call drm_framebuffer_remove from a
> workqueue, which is unaffected by signals.
>
> Changes since v1:
> - Add comment.
> Changes since v2:
> - Add fastpath for refcount = 1. (danvet)
> Changes since v3:
> - Rebased.
> - Restore lastclose framebuffer removal too.
>
> Cc: stable@vger.kernel.org #v4.4+
> Fixes: 13803132818c ("drm/core: Preserve the framebuffer after removing it.")
> Testcase: kms_rmfb_basic
> References: https://lists.freedesktop.org/archives/dri-devel/2016-March/102876.html
> Cc: Thomas Hellstrom <thellstrom@vmware.com>
> Cc: David Herrmann <dh.herrmann@gmail.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Tested-by: Thomas Hellstrom <thellstrom@vmware.com> #v3

Can't be 100% since apparently eDP regressed a lot recently for me, but 
seems to be effective in getting rid of stale planes in my test cases.

Tested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

> ---
>   drivers/gpu/drm/drm_crtc.c | 63 ++++++++++++++++++++++++++++++++++++++++------
>   1 file changed, 56 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index 9626a0cc050a..9a3d17b70091 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -3440,6 +3440,24 @@ int drm_mode_addfb2(struct drm_device *dev,
>   	return 0;
>   }
>
> +struct drm_mode_rmfb_work {
> +	struct work_struct work;
> +	struct list_head fbs;
> +};
> +
> +static void drm_mode_rmfb_work_fn(struct work_struct *w)
> +{
> +	struct drm_mode_rmfb_work *arg = container_of(w, typeof(*arg), work);
> +
> +	while (!list_empty(&arg->fbs)) {
> +		struct drm_framebuffer *fb =
> +			list_first_entry(&arg->fbs, typeof(*fb), filp_head);
> +
> +		list_del_init(&fb->filp_head);
> +		drm_framebuffer_remove(fb);
> +	}
> +}
> +
>   /**
>    * drm_mode_rmfb - remove an FB from the configuration
>    * @dev: drm device for the ioctl
> @@ -3480,12 +3498,29 @@ int drm_mode_rmfb(struct drm_device *dev,
>   	list_del_init(&fb->filp_head);
>   	mutex_unlock(&file_priv->fbs_lock);
>
> -	/* we now own the reference that was stored in the fbs list */
> -	drm_framebuffer_unreference(fb);
> -
>   	/* drop the reference we picked up in framebuffer lookup */
>   	drm_framebuffer_unreference(fb);
>
> +	/*
> +	 * we now own the reference that was stored in the fbs list
> +	 *
> +	 * drm_framebuffer_remove may fail with -EINTR on pending signals,
> +	 * so run this in a separate stack as there's no way to correctly
> +	 * handle this after the fb is already removed from the lookup table.
> +	 */
> +	if (drm_framebuffer_read_refcount(fb) > 1) {
> +		struct drm_mode_rmfb_work arg;
> +
> +		INIT_WORK_ONSTACK(&arg.work, drm_mode_rmfb_work_fn);
> +		INIT_LIST_HEAD(&arg.fbs);
> +		list_add_tail(&fb->filp_head, &arg.fbs);
> +
> +		schedule_work(&arg.work);
> +		flush_work(&arg.work);
> +		destroy_work_on_stack(&arg.work);
> +	} else
> +		drm_framebuffer_unreference(fb);
> +
>   	return 0;
>
>   fail_unref:
> @@ -3635,7 +3670,6 @@ out_err1:
>   	return ret;
>   }
>
> -
>   /**
>    * drm_fb_release - remove and free the FBs on this file
>    * @priv: drm file for the ioctl
> @@ -3650,6 +3684,9 @@ out_err1:
>   void drm_fb_release(struct drm_file *priv)
>   {
>   	struct drm_framebuffer *fb, *tfb;
> +	struct drm_mode_rmfb_work arg;
> +
> +	INIT_LIST_HEAD(&arg.fbs);
>
>   	/*
>   	 * When the file gets released that means no one else can access the fb
> @@ -3662,10 +3699,22 @@ void drm_fb_release(struct drm_file *priv)
>   	 * at it any more.
>   	 */
>   	list_for_each_entry_safe(fb, tfb, &priv->fbs, filp_head) {
> -		list_del_init(&fb->filp_head);
> +		if (drm_framebuffer_read_refcount(fb) > 1) {
> +			list_move_tail(&fb->filp_head, &arg.fbs);
> +		} else {
> +			list_del_init(&fb->filp_head);
>
> -		/* This drops the fpriv->fbs reference. */
> -		drm_framebuffer_unreference(fb);
> +			/* This drops the fpriv->fbs reference. */
> +			drm_framebuffer_unreference(fb);
> +		}
> +	}
> +
> +	if (!list_empty(&arg.fbs)) {
> +		INIT_WORK_ONSTACK(&arg.work, drm_mode_rmfb_work_fn);
> +
> +		schedule_work(&arg.work);
> +		flush_work(&arg.work);
> +		destroy_work_on_stack(&arg.work);
>   	}
>   }
>
>
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