Re: [Qemu-devel] [PATCH v2 3/3] char-socket: Lock tcp_chr_disconnect() and socket_reconnect_timeout()

[Daniel P . Berrangé]

On Fri, Feb 22, 2019 at 01:59:12PM +0200, Alberto Garcia wrote:
> There's a race condition in which the tcp_chr_read() ioc handler can
> close a connection that is being written to from another thread.
> 
> Running iotest 136 in a loop triggers this problem and crashes QEMU.
> 
>  (gdb) bt
>  #0  0x00005558b842902d in object_get_class (obj=0x0) at qom/object.c:860
>  #1  0x00005558b84f92db in qio_channel_writev_full (ioc=0x0, 
> iov=0x7ffc355decf0, niov=1, fds=0x0, nfds=0, errp=0x0) at io/channel.c:76
>  #2  0x00005558b84e0e9e in io_channel_send_full (ioc=0x0, buf=0x5558baf5beb0, 
> len=138, fds=0x0, nfds=0) at chardev/char-io.c:123
>  #3  0x00005558b84e4a69 in tcp_chr_write (chr=0x5558ba460380, 
> buf=0x5558baf5beb0 "...", len=138) at chardev/char-socket.c:135
>  #4  0x00005558b84dca55 in qemu_chr_write_buffer (s=0x5558ba460380, 
> buf=0x5558baf5beb0 "...", len=138, offset=0x7ffc355dedd0, write_all=false) at 
> chardev/char.c:112
>  #5  0x00005558b84dcbc2 in qemu_chr_write (s=0x5558ba460380, 
> buf=0x5558baf5beb0 "...", len=138, write_all=false) at chardev/char.c:147
>  #6  0x00005558b84dfb26 in qemu_chr_fe_write (be=0x5558ba476610, 
> buf=0x5558baf5beb0 "...", len=138) at chardev/char-fe.c:42
>  #7  0x00005558b8088c86 in monitor_flush_locked (mon=0x5558ba476610) at 
> monitor.c:406
>  #8  0x00005558b8088e8c in monitor_puts (mon=0x5558ba476610, 
> str=0x5558ba921e49 "") at monitor.c:449
>  #9  0x00005558b8089178 in qmp_send_response (mon=0x5558ba476610, 
> rsp=0x5558bb161600) at monitor.c:498
>  #10 0x00005558b808920c in monitor_qapi_event_emit 
> (event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:526
>  #11 0x00005558b8089307 in monitor_qapi_event_queue_no_reenter 
> (event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:551
>  #12 0x00005558b80896c0 in qapi_event_emit (event=QAPI_EVENT_SHUTDOWN, 
> qdict=0x5558bb161600) at monitor.c:626
>  #13 0x00005558b855f23b in qapi_event_send_shutdown (guest=false, 
> reason=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at qapi/qapi-events-run-state.c:43
>  #14 0x00005558b81911ef in qemu_system_shutdown 
> (cause=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at vl.c:1837
>  #15 0x00005558b8191308 in main_loop_should_exit () at vl.c:1885
>  #16 0x00005558b819140d in main_loop () at vl.c:1924
>  #17 0x00005558b8198c84 in main (argc=18, argv=0x7ffc355df3f8, 
> envp=0x7ffc355df490) at vl.c:4665
> 
> This patch adds a lock to protect tcp_chr_disconnect() and
> socket_reconnect_timeout()

Can you think of any way to test this in the unit tests ?  I can understand
if its too difficult but just curious if there's any viable option ?

> diff --git a/chardev/char-socket.c b/chardev/char-socket.c
> index 4fcdd8aedd..67c2eeac6d 100644
> --- a/chardev/char-socket.c
> +++ b/chardev/char-socket.c
> @@ -172,7 +172,9 @@ static int tcp_chr_write(Chardev *chr, const uint8_t 
> *buf, int len)
>  
>          if (ret < 0 && errno != EAGAIN) {
>              if (tcp_chr_read_poll(chr) <= 0) {
> +                qemu_mutex_unlock(&chr->chr_write_lock);
>                  tcp_chr_disconnect(chr);
> +                qemu_mutex_lock(&chr->chr_write_lock);

The idea of unlock & relocking here, just so tcp_chr_disconnect can
also acquire locks, doesn't make me too happy. This is really an
anti-pattern to me.

I think we should rename the existing method to be
tcp_chr_disconnect_locked() and document that it must only
be called with the write lock held, and call that from
here.

Then introduce a new wrapper for all the other callers to
carry on using which does the locking

 static void tcp_chr_disconnect(Chardev *chr)
 {
    qemu_mutex_lock(&chr->chr_write_lock);
    tcp_chr_disconnect_locked(chr);
    qemu_mutex_unlock(&chr->chr_write_lock);
 }
  
Regards,
Daniel
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