Re: [Qemu-devel] vhost-user devices work with chardev from different threads

[Marc-André Lureau]

Hi

On Mon, Oct 29, 2018 at 5:35 PM Yury Kotov <address@hidden> wrote:
>
>
>
> 29.10.2018, 09:46, "Marc-André Lureau" <address@hidden>:
> > Hi
> >
> > On Mon, Oct 22, 2018 at 5:24 PM Yury Kotov <address@hidden> wrote:
> >
> >>  Hi,
> >>
> >>  I examined vhost-user devices and found some chardev using strangeness.
> >>
> >>  Is it ok, that vhost-user's set_status do sync chardev io ops from KVM 
> >> thread?
> >>  It seems that chardev doesn't support working with different threads.
> >>
> >>  For example, I think such race is possible (two simultaneous events):
> >>  1) Vhost-user server was killed (main thread will handle G_IO_HUP),
> >>  2) Virtio-pci bus handles guest driver's set_status command.
> >>     Some io op from vhost-user backend will fail because of 1).
> >>
> >>  So, it is possible to enter tcp_chr_disconnect twice from the main thread 
> >> and
> >>  from KVM thread.
> >>
> >>  Backtrace examples:
> >>
> >>  KVM thread (handle set_status of guest):
> >>  tcp_chr_disconnect
> >>  tcp_chr_write (cc->chr_write)
> >>  qemu_chr_write_buffer
> >>  qemu_chr_write
> >>  qemu_chr_fe_write_all
> >>  vhost_user_write
> >>  vhost_user_set_mem_table (hdev->vhost_ops->vhost_set_mem_table)
> >>  vhost_dev_start
> >>  vhost_user_blk_start (or another vhost-user device)
> >>  vhost_user_blk_set_status (vdc->set_status)
> >>
> >>  Main thread (handle vhost server disconnect):
> >>  tcp_chr_disconnect
> >>  tcp_chr_hup
> >>  g_main_context_dispatch
> >>  glib_pollfds_poll
> >>  os_host_main_loop_wait
> >>  main_loop_wait
> >>  main_loop
> >>  main
> >>
> >>  Is it really a problem or do I misunderstand?
> >
> > I think you are correct, it's a problem. Are you working on a
> > solution? (either using the chardev lock, or perhaps relying on main
> > thread HUP handler only, or a combination of both approaches)
>
> So, chardev is thread safe by design? I thought it is vhost's problem...
> Anyway I plan to fix that.

chardev is not thread-safe in general, only the write path (assuming
everything else, like lifecycle, is done safely by the user).

>
> >
> > thanks
> >
> > --
> > Marc-André Lureau



-- 
Marc-André Lureau