linux-kernel - Re: BUG: 'list_empty(&vgdev->free

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Date:   Tue, 15 Nov 2016 09:46:28 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Subject: Re: BUG: 'list_empty(&vgdev->free_vbufs)' is true!

On Fr, 2016-11-11 at 17:28 +0100, Jiri Slaby wrote:
> On 11/09/2016, 09:01 AM, Gerd Hoffmann wrote:
> > On Di, 2016-11-08 at 22:37 +0200, Michael S. Tsirkin wrote:
> >> On Mon, Nov 07, 2016 at 09:43:24AM +0100, Jiri Slaby wrote:
> >>> Hi,
> >>>
> >>> I can relatively easily reproduce this bug:
> > 
> > How?
> 
> Run dmesg -w in the qemu window (virtio_gpu) to see a lot of output.

fbcon?  Or xorg/wayland with terminal app?

> Run pps [1] without exit(0); on e.g. serial console.
> Wait a bit. The lot of output causes the BUG.
> 
> [1] https://github.com/jirislaby/collected_sources/blob/master/pps.c
> 
> >>> BUG: 'list_empty(&vgdev->free_vbufs)' is true!
> > 
> >> The following might be helpful for debugging - if kernel still will
> >> not stop panicing, we are looking at some kind
> >> of memory corruption.
> > 
> > Looking carefully through the code I think it isn't impossible to
> > trigger this, but you need for that:
> > 
> >   (1) command queue full (quite possible),
> >   (2) cursor queue full too (unlikely), and
> >   (3) multiple threads trying to submit commands and waiting for free
> >       space in the command queue (possible with virgl enabled).
> 
> I use -vga virtio with no -display option, so no virtgl, I suppose:
> [drm] virgl 3d acceleration not available
> 
> > Do things improve if you allocate some extra bufs?
> > 
> >  int virtio_gpu_alloc_vbufs(struct virtio_gpu_device *vgdev)
> >  {
> >         struct virtio_gpu_vbuffer *vbuf;
> > -       int i, size, count = 0;
> > +       int i, size, count = 16;
> 
> This seems to help.
> 
> thanks,