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

[Yury Kotov]

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?

Thanks,
Yury