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Re: [Qemu-devel] vhost-user devices work with chardev from different thr
From: |
Yury Kotov |
Subject: |
Re: [Qemu-devel] vhost-user devices work with chardev from different threads |
Date: |
Wed, 07 Nov 2018 20:03:28 +0300 |
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
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