block-backend: fix edge case in bdrv_next() where BDS associated to BB changes

block-backend: fix edge case in bdrv_next() where BDS associated to BB changes

The old_bs variable in bdrv_next() is currently determined by looking
at the old block backend. However, if the block graph changes before
the next bdrv_next() call, it might be that the associated BDS is not
the same that was referenced previously. In that case, the wrong BDS
is unreferenced, leading to an assertion failure later:

> bdrv_unref: Assertion `bs->refcnt > 0' failed.

In particular, this can happen in the context of bdrv_flush_all(),
when polling for bdrv_co_flush() in the generated co-wrapper leads to
a graph change (for example with a stream block job [0]).

A racy reproducer:

> #!/bin/bash
> rm -f /tmp/backing.qcow2
> rm -f /tmp/top.qcow2
> ./qemu-img create /tmp/backing.qcow2 -f qcow2 64M
> ./qemu-io -c "write -P42 0x0 0x1" /tmp/backing.qcow2
> ./qemu-img create /tmp/top.qcow2 -f qcow2 64M -b /tmp/backing.qcow2 -F qcow2
> ./qemu-system-x86_64 --qmp stdio \
> --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/top.qcow2 \
> <<EOF
> {"execute": "qmp_capabilities"}
> {"execute": "block-stream", "arguments": { "job-id": "stream0", "device": "node0" } }
> {"execute": "quit"}
> EOF

[0]:

> #0  bdrv_replace_child_tran (child=..., new_bs=..., tran=...)
> #1  bdrv_replace_node_noperm (from=..., to=..., auto_skip=..., tran=..., errp=...)
> #2  bdrv_replace_node_common (from=..., to=..., auto_skip=..., detach_subchain=..., errp=...)
> #3  bdrv_drop_filter (bs=..., errp=...)
> #4  bdrv_cor_filter_drop (cor_filter_bs=...)
> #5  stream_prepare (job=...)
> #6  job_prepare_locked (job=...)
> #7  job_txn_apply_locked (fn=..., job=...)
> #8  job_do_finalize_locked (job=...)
> #9  job_exit (opaque=...)
> #10 aio_bh_poll (ctx=...)
> #11 aio_poll (ctx=..., blocking=...)
> #12 bdrv_poll_co (s=...)
> #13 bdrv_flush (bs=...)
> #14 bdrv_flush_all ()
> #15 do_vm_stop (state=..., send_stop=...)
> #16 vm_shutdown ()

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20240322095009.346989-3-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>