Breaking on the two check functions and the calling one to see where
things break:
b virtnet_send_command
# virtqueue_get_buf gets hit by __do_softirq -> napi_poll -> virtnet_poll ->
virtnet_receive -> virtqueue_get_buf all the time.
Need to keep that disabled and step INTO from virtnet_send_command.
b virtqueue_get_buf
b virtqueue_is_broken
Here is what we see in the two checkers then
virtqueue_get_buf (_vq=0xffff8801b6b7d000, len=0xffff8801b7f17b64) at
/build/linux-XwpX40/linux-4.4.0/drivers/virtio/virtio_ring.c:478
p *(_vq)
$12 = {list = {next = 0xffff8801b69c8b00, prev = 0xffff8801b640d000}, callback
= 0x0 <irq_stack_union>, name = 0xffffffff81d094d7 "control", vdev =
0xffff8801b69c8800,
index = 8, num_free = 63, priv = 0x1c010}
if (unlikely(!vq->data[i])) {
BAD_RING(vq, "id %u is not a head!\n", i);
return NULL;
}
ret = vq->data[i];
[...]
return ret;
So this should for sure be valid when returning or we would see the BAD_RING.
But then it is looping on after returning on
while !virtqueue_get_buf(vi->cvq, &tmp) && !virtqueue_is_broken(vi->cvq)
So we "should be" (tm) safe to assume that we always get a good buffer
back, but then lack one?
Too much is optimized out by default to take a much deeper look.
I need to understand more what happens there, so I'm going to recompile the
kernel with extra stuff, more debug and less optimization.
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https://bugs.launchpad.net/bugs/1570195
Title:
Net tools cause kernel soft lockup after DPDK touched VirtIO-pci
devices
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