Re: [Qemu-devel] [PATCH] monitor: avoid potential dead-lock when cleaning up

[Markus Armbruster]

Marc-André Lureau <address@hidden> writes:

> When a monitor is connected to a Spice chardev, the monitor cleanup
> can dead-lock:
>
>  #0  0x00007f43446637fd in __lll_lock_wait () at /lib64/libpthread.so.0
>  #1  0x00007f434465ccf4 in pthread_mutex_lock () at /lib64/libpthread.so.0
>  #2  0x0000556dd79f22ba in qemu_mutex_lock_impl (mutex=0x556dd81c9220 
> <monitor_lock>, file=0x556dd7ae3648 "/home/elmarco/src/qq/monitor.c", 
> line=645) at /home/elmarco/src/qq/util/qemu-thread-posix.c:66
>  #3  0x0000556dd7431bd5 in monitor_qapi_event_queue 
> (event=QAPI_EVENT_SPICE_DISCONNECTED, qdict=0x556dd9abc850, 
> errp=0x7fffb7bbddd8) at /home/elmarco/src/qq/monitor.c:645
>  #4  0x0000556dd79d476b in qapi_event_send_spice_disconnected 
> (server=0x556dd98ee760, client=0x556ddaaa8560, errp=0x556dd82180d0 
> <error_abort>) at qapi/qapi-events-ui.c:149
>  #5  0x0000556dd7870fc1 in channel_event (event=3, info=0x556ddad1b590) at 
> /home/elmarco/src/qq/ui/spice-core.c:235
>  #6  0x00007f434560a6bb in reds_handle_channel_event (reds=<optimized out>, 
> event=3, info=0x556ddad1b590) at reds.c:316
>  #7  0x00007f43455f393b in main_dispatcher_self_handle_channel_event 
> (info=0x556ddad1b590, event=3, self=0x556dd9a7d8c0) at main-dispatcher.c:197
>  #8  0x00007f43455f393b in main_dispatcher_channel_event 
> (self=0x556dd9a7d8c0, address@hidden, info=0x556ddad1b590) at 
> main-dispatcher.c:197
>  #9  0x00007f4345612833 in red_stream_push_channel_event (address@hidden, 
> address@hidden) at red-stream.c:414
>  #10 0x00007f434561286b in red_stream_free (s=0x556ddae2ef40) at 
> red-stream.c:388
>  #11 0x00007f43455f9ddc in red_channel_client_finalize 
> (object=0x556dd9bb21a0) at red-channel-client.c:347
>  #12 0x00007f434b5f9fb9 in g_object_unref () at /lib64/libgobject-2.0.so.0
>  #13 0x00007f43455fc212 in red_channel_client_push (rcc=0x556dd9bb21a0) at 
> red-channel-client.c:1341
>  #14 0x0000556dd76081ba in spice_port_set_fe_open (chr=0x556dd9925e20, 
> fe_open=0) at /home/elmarco/src/qq/chardev/spice.c:241
>  #15 0x0000556dd796d74a in qemu_chr_fe_set_open (be=0x556dd9a37c00, 
> fe_open=0) at /home/elmarco/src/qq/chardev/char-fe.c:340
>  #16 0x0000556dd796d4d9 in qemu_chr_fe_set_handlers (b=0x556dd9a37c00, 
> fd_can_read=0x0, fd_read=0x0, fd_event=0x0, be_change=0x0, opaque=0x0, 
> context=0x0, set_open=true) at /home/elmarco/src/qq/chardev/char-fe.c:280
>  #17 0x0000556dd796d359 in qemu_chr_fe_deinit (b=0x556dd9a37c00, del=false) 
> at /home/elmarco/src/qq/chardev/char-fe.c:233
>  #18 0x0000556dd7432240 in monitor_data_destroy (mon=0x556dd9a37c00) at 
> /home/elmarco/src/qq/monitor.c:786
>  #19 0x0000556dd743b968 in monitor_cleanup () at 
> /home/elmarco/src/qq/monitor.c:4683
>  #20 0x0000556dd75ce776 in main (argc=3, argv=0x7fffb7bbe458, 
> envp=0x7fffb7bbe478) at /home/elmarco/src/qq/vl.c:4660
>
> Because spice code tries to emit a "disconnected" signal on the
> monitors. Fix this situation by tightening the monitor lock time to
> the monitor list removal.
>
> Signed-off-by: Marc-André Lureau <address@hidden>

Do you think this should go into 3.0?

> ---
>  monitor.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
>
> diff --git a/monitor.c b/monitor.c
> index 0fa0910a2a..a16a6c5311 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -4702,8 +4702,6 @@ void monitor_init(Chardev *chr, int flags)
>  
>  void monitor_cleanup(void)
>  {
> -    Monitor *mon, *next;
> -
>      /*
>       * We need to explicitly stop the I/O thread (but not destroy it),
>       * clean up the monitor resources, then destroy the I/O thread since
> @@ -4719,14 +4717,24 @@ void monitor_cleanup(void)
>      monitor_qmp_bh_responder(NULL);
>  
>      /* Flush output buffers and destroy monitors */
> -    qemu_mutex_lock(&monitor_lock);
> -    QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
> -        QTAILQ_REMOVE(&mon_list, mon, entry);
> +    do {

for (;;), please.

> +        Monitor *mon;
> +
> +        qemu_mutex_lock(&monitor_lock);
> +        mon = QTAILQ_FIRST(&mon_list);
> +        if (mon) {
> +            QTAILQ_REMOVE(&mon_list, mon, entry);
> +        }
> +        qemu_mutex_unlock(&monitor_lock);
> +
> +        if (!mon) {
> +            break;
> +        }
> +
>          monitor_flush(mon);
>          monitor_data_destroy(mon);
>          g_free(mon);
> -    }
> -    qemu_mutex_unlock(&monitor_lock);
> +    } while (true);
>  
>      /* QEMUBHs needs to be deleted before destroying the I/O thread */
>      qemu_bh_delete(qmp_dispatcher_bh);

Iterating safely over a list protected by a lock should be easier than
that.  Sad.

Hmm, what about this:

diff --git a/monitor.c b/monitor.c
index 77861e96af..4a23f6c7bc 100644
--- a/monitor.c
+++ b/monitor.c
@@ -4721,9 +4721,11 @@ void monitor_cleanup(void)
     qemu_mutex_lock(&monitor_lock);
     QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
         QTAILQ_REMOVE(&mon_list, mon, entry);
+        qemu_mutex_unlock(&monitor_lock);
         monitor_flush(mon);
         monitor_data_destroy(mon);
         g_free(mon);
+        qemu_mutex_lock(&monitor_lock);
     }
     qemu_mutex_unlock(&monitor_lock);