linux-kernel - Re: [PATCH v2 4/4] drm/qxl: use qxl pin function

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Date:   Tue, 29 Sep 2020 12:53:00 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org, Dave Airlie <airlied@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Huang Rui <ray.huang@....com>,
        Christian König <christian.koenig@....com>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <virtualization@...ts.linux-foundation.org>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <spice-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] drm/qxl: use qxl pin function

On Tue, Sep 29, 2020 at 11:51:15AM +0200, Gerd Hoffmann wrote:
> Otherwise ttm throws a WARN because we try to pin without a reservation.
> 
> Fixes: 9d36d4320462 ("drm/qxl: switch over to the new pin interface")
> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> ---
>  drivers/gpu/drm/qxl/qxl_object.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/qxl/qxl_object.c b/drivers/gpu/drm/qxl/qxl_object.c
> index d3635e3e3267..eb45267d51db 100644
> --- a/drivers/gpu/drm/qxl/qxl_object.c
> +++ b/drivers/gpu/drm/qxl/qxl_object.c
> @@ -145,7 +145,7 @@ int qxl_bo_create(struct qxl_device *qdev,
>  		return r;
>  	}
>  	if (pinned)
> -		ttm_bo_pin(&bo->tbo);
> +		qxl_bo_pin(bo);

I think this is now after ttm_bo_init, and at that point the object is
visible to lru users and everything. So I do think you need to grab locks
here instead of just incrementing the pin count alone.

It's also I think a bit racy, since ttm_bo_init drops the lock, so someone
might have snuck in and evicted the object already.

I think what you need is to call ttm_bo_init_reserved, then ttm_bo_pin,
then ttm_bo_unreserve, all explicitly.
-Daniel

>  	*bo_ptr = bo;
>  	return 0;
>  }
> -- 
> 2.27.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch