Re: [Qemu-devel] [PATCH] vhost-user: return if no net clients found

[Marc-André Lureau]

Hi

On Thu, Sep 1, 2016 at 4:00 PM Chen Hanxiao <address@hidden> wrote:

>
> Hi, here is the backtrace:
>
> #0  net_vhost_user_event (opaque=0x7fc2f6893be0, event=5) at
> net/vhost-user.c:196
> #1  0x00007fc2f4ebfb2b in tcp_chr_disconnect (chr=0x7fc2f68cc400) at
> qemu-char.c:2837
> #2  0x00007fc2f4ebfba9 in tcp_chr_sync_read (chr=0x7fc2f68cc400,
> buf=<optimized out>, len=<optimized out>) at qemu-char.c:2888
> #3  0x00007fc2f4ec106d in qemu_chr_fe_read_all (s=0x7fc2f68cc400,
> address@hidden "", address@hidden) at qemu-char.c:264
> #4  0x00007fc2f4f9a43a in net_vhost_user_watch (chan=<optimized out>,
> cond=<optimized out>, opaque=<optimized out>) at net/vhost-user.c:190
> #5  0x00007fc2f321999a in g_main_context_dispatch () from
> /lib64/libglib-2.0.so.0
> #6  0x00007fc2f4fd8fe8 in glib_pollfds_poll () at main-loop.c:209
> #7  os_host_main_loop_wait (timeout=<optimized out>) at main-loop.c:254
> #8  main_loop_wait (nonblocking=<optimized out>) at main-loop.c:503
> #9  0x00007fc2f4dd7b1e in main_loop () at vl.c:1818
> #10 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized
> out>) at vl.c:4394
>
>
> Program received signal SIGSEGV, Segmentation fault.
> net_vhost_user_event (opaque=0x7fc2f6893be0, event=5) at
> net/vhost-user.c:207
> 207    trace_vhost_user_event(s->chr->label, event);
>
>
thanks for the backtrace, that helps

However, I fail to understand how that can happen, as there has to be at
least one net_client to start qemu with vhost-user and that callback must
have at least the first netclient still around because the opaque pointer
is shared with the netclient struct. So it looks like something destroyed
the netclient before the callback, and in this case, the opaque pointer is
invalid, and things are going all wrong. But it can't be host-net-remove,
since the net-client is not on a registered hub.

Could you try to find a simple reproducer using qemu only?

thanks

-- 
Marc-André Lureau