hotplug issue of vhost-user-blk

[Li Feng <fengli@smartx.com>]

Hi all,

Hotplug of vhost-user-blk doesn't not work in qemu master branch and
all previous version.

The action I insert a vhost-user-blk disk is:
(qemu) chardev-add socket,id=spdk_vhost_blk2,path=/vhost-blk.0,reconnect=1
(qemu) device_add
vhost-user-blk-pci,chardev=spdk_vhost_blk2,id=spdk_vhost_blk2,num-queues=4

Until here, it's well.

Then I unplug it from qemu:
(qemu) device_del spdk_vhost_blk2
(qemu) chardev-remove spdk_vhost_blk2
Error: Chardev 'spdk_vhost_blk2' is busy

The related code is here:
qmp_chardev_remove
    -> qemu_chr_is_busy
    -> object_unparent(OBJECT(chr));

 330 static bool qemu_chr_is_busy(Chardev *s)
 331 {
 332     if (CHARDEV_IS_MUX(s)) {
 333         MuxChardev *d = MUX_CHARDEV(s);
 334         return d->mux_cnt >= 0;
 335     } else {
 336         return s->be != NULL;
 337     }
 338 }

My question is:
1. s->be is set to NULL when qemu_chr_fe_deinit is called.
However, the qmp_chardev_remove is blocked at qemu_chr_is_busy check,
then the object_unparent will not be called.
2. Is there a path that device_del will trigger the s->be that been set to NULL?

How should I fix this issue?
I have tested that comment the qemu_chr_is_busy works well.

Thanks in advance.

Feng Li

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