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

[Yury Kotov]

Hi,

I was wrong... Described problem with vhost-user is not possible.
It cannot enter tcp_chr_disconnect twice because of qemu global mutex (BQL).
Thus chardev event handler (main loop) and vhost_user_blk_set_status (VCPU) are
working under BQL.

30.10.2018, 20:12, "Marc-André Lureau" <address@hidden>:
> 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