Re: BUG: 'list_empty(&vgdev->free_vbufs)' is true!

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 07, 2016 at 09:43:24AM +0100, Jiri Slaby wrote:
> Hi,
> 
> I can relatively easily reproduce this bug:
> BUG: 'list_empty(&vgdev->free_vbufs)' is true!
> ------------[ cut here ]------------
> kernel BUG at /home/latest/linux/drivers/gpu/drm/virtio/virtgpu_vq.c:130!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> Modules linked in:
> CPU: 1 PID: 355 Comm: kworker/1:2 Not tainted 4.9.0-rc2-next-20161028+ #32
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> rel-1.9.3-0-ge2fc41e-prebuilt.qemu-project.org 04/01/2014
> Workqueue: events drm_fb_helper_dirty_work
> task: ffff88007b124980 task.stack: ffff88007b8a0000
> RIP: 0010:virtio_gpu_get_vbuf+0x32e/0x630
> RSP: 0018:ffff88007b8a78c0 EFLAGS: 00010286
> RAX: 000000000000002e RBX: 1ffff1000f714f1d RCX: 0000000000000000
> RDX: 000000000000002e RSI: 0000000000000001 RDI: ffffed000f714f0e
> RBP: ffff88007b8a7970 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000002 R11: 0000000000000001 R12: 0000000000000030
> R13: ffff88007caeaba8 R14: 0000000000000018 R15: ffff88007cae0000
> FS:  0000000000000000(0000) GS:ffff88007dc80000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000601028 CR3: 000000007740d000 CR4: 00000000000006e0
> Call Trace:
> Code: df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 bb 01 00 00 4c 89 69 e8
> eb 9e 48 c7 c6 e0 d2 d1 83 48 c7 c7 20 d3 d1 83 e8 6c fb 04 ff <0f> 0b
> 48 c7 c7 a0 fb b0 85 e8 09 95 86 ff 48 c7 c6 c0 d3 d1 83
> RIP: virtio_gpu_get_vbuf+0x32e/0x630 RSP: ffff88007b8a78c0
> 
> 
> There is no stacktrace, as the kernel starts panicing all over the place
> during its generation. Any ideas?
> 
> thanks,

CC maintainers.

The following might be helpful for debugging - if kernel still will
not stop panicing, we are looking at some kind
of memory corruption.


diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
index 5a0f8a7..d5e1e72 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -127,7 +127,11 @@ virtio_gpu_get_vbuf(struct virtio_gpu_device *vgdev,
 	struct virtio_gpu_vbuffer *vbuf;
 
 	spin_lock(&vgdev->free_vbufs_lock);
-	BUG_ON(list_empty(&vgdev->free_vbufs));
+	WARN_ON(list_empty(&vgdev->free_vbufs));
+	if (list_empty(&vgdev->free_vbufs)) {
+		spin_unlock(&vgdev->free_vbufs_lock);
+		return ERR_PTR(-EINVAL);
+	}
 	vbuf = list_first_entry(&vgdev->free_vbufs,
 				struct virtio_gpu_vbuffer, list);
 	list_del(&vbuf->list);