Re: hotplug issue of vhost-user-blk

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

On Wed, Apr 08, 2020 at 10:25:42AM +0800, Li Feng wrote:
> 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

Pan Nengyuan (cc'd) fixed unplug recently so I expect it
worked for him after the patch ...

> 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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