[Qemu-devel] Possible SPICE/QEMU deadlock on fast client reconnect

["Daniel P. Berrange" <berrange@redhat.com>]

In testing my patches for 'add_client' support with SPICE, I noticed
deadlock in the QEMU/SPICE code. It only happens if I close a SPICE
client and then immediately reconnect within about 1 second. If I
wait a couple of seconds before reconnecting the SPICE client I don't
see the deadlock.

I'm using QEMU & SPICE git master, and GDB has this to say

(gdb) thread apply all bt

Thread 3 (Thread 0x7f269e142700 (LWP 17233)):
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:165
#1  0x00000000004e80e9 in qemu_cond_wait (cond=<optimized out>, mutex=<optimized out>)
    at qemu-thread-posix.c:113
#2  0x0000000000556469 in qemu_kvm_wait_io_event (env=<optimized out>)
    at /home/berrange/src/virt/qemu/cpus.c:626
#3  qemu_kvm_cpu_thread_fn (arg=0x29217a0) at /home/berrange/src/virt/qemu/cpus.c:661
#4  0x0000003a83207b31 in start_thread (arg=0x7f269e142700) at pthread_create.c:305
#5  0x0000003a82edfd2d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:115

Thread 2 (Thread 0x7f26822fc700 (LWP 17234)):
#0  __lll_lock_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:140
#1  0x0000003a83209ad6 in _L_lock_752 () from /lib64/libpthread.so.0
#2  0x0000003a832099d7 in __pthread_mutex_lock (mutex=0x1182f60) at pthread_mutex_lock.c:65
#3  0x00000000004e7ec9 in qemu_mutex_lock (mutex=<optimized out>) at qemu-thread-posix.c:54
#4  0x0000000000507df5 in channel_event (event=3, info=0x2a3c610) at ui/spice-core.c:234
#5  0x00007f269f77be87 in reds_stream_free (s=0x2a3c590) at reds.c:4073
#6  0x00007f269f75b64f in red_channel_client_disconnect (rcc=0x7f267c069ec0) at red_channel.c:961
#7  0x00007f269f75a131 in red_peer_handle_incoming (handler=0x7f267c06a5a0, stream=0x2a3c590)
    at red_channel.c:150
#8  red_channel_client_receive (rcc=0x7f267c069ec0) at red_channel.c:158
#9  0x00007f269f761fbc in handle_channel_events (in_listener=0x7f267c06a5f8, events=<optimized out>)
    at red_worker.c:9566
#10 0x00007f269f776672 in red_worker_main (arg=<optimized out>) at red_worker.c:10813
#11 0x0000003a83207b31 in start_thread (arg=0x7f26822fc700) at pthread_create.c:305
#12 0x0000003a82edfd2d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:115

Thread 1 (Thread 0x7f269f72e9c0 (LWP 17232)):
#0  0x0000003a8320e01d in read () at ../sysdeps/unix/syscall-template.S:82
#1  0x00007f269f75daaa in receive_data (n=4, in_buf=0x7fffe7a5a02c, fd=18) at red_worker.h:150
#2  read_message (message=0x7fffe7a5a02c, fd=18) at red_worker.h:163
#3  red_dispatcher_disconnect_cursor_peer (rcc=0x7f267c0c0f60) at red_dispatcher.c:157
#4  0x00007f269f75c20d in red_client_destroy (client=0x2a35400) at red_channel.c:1189
#5  0x00007f269f778335 in reds_client_disconnect (client=0x2a35400) at reds.c:624
---Type <return> to continue, or q <return> to quit---
#6  0x00007f269f75a131 in red_peer_handle_incoming (handler=0x2a35b50, stream=0x2b037d0) at red_channel.c:150
#7  red_channel_client_receive (rcc=0x2a35470) at red_channel.c:158
#8  0x00007f269f75b1e8 in red_channel_client_event (fd=<optimized out>, event=<optimized out>, 
    data=<optimized out>) at red_channel.c:774
#9  0x000000000045e561 in fd_trampoline (chan=<optimized out>, cond=<optimized out>, opaque=<optimized out>)
    at iohandler.c:97
#10 0x0000003a84e427ed in g_main_dispatch (context=0x27fc510) at gmain.c:2441
#11 g_main_context_dispatch (context=0x27fc510) at gmain.c:3014
#12 0x00000000004c3da3 in glib_select_poll (err=false, xfds=0x7fffe7a5a2e0, wfds=0x7fffe7a5a260, rfds=
    0x7fffe7a5a1e0) at /home/berrange/src/virt/qemu/vl.c:1495
#13 main_loop_wait (nonblocking=<optimized out>) at /home/berrange/src/virt/qemu/vl.c:1541
#14 0x000000000040fdd1 in main_loop () at /home/berrange/src/virt/qemu/vl.c:1570
#15 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>)
    at /home/berrange/src/virt/qemu/vl.c:3563
(gdb) 


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