[Spice-devel] [Qemu-devel] [PATCH] monitor: Protect outbuf from concurrent access

[Gerd Hoffmann]

   Hi,

>> After some investigation, I found out that the problem is that different
>> SPICE threads are calling monitor functions (such as
>> monitor_protocol_event()) in parallel which causes concurrent access
>> to the monitor's internal buffer outbuf[].

[ adding spice-list to Cc, see qemu-devel for the rest of the thread ]

spice isn't supposed to do that.

/me just added a assert in channel_event() and saw it trigger in display 
channel disconnects.

#0  0x0000003ceba32a45 in raise () from /lib64/libc.so.6
#1  0x0000003ceba34225 in abort () from /lib64/libc.so.6
#2  0x0000003ceba2b9d5 in __assert_fail () from /lib64/libc.so.6
#3  0x0000000000503759 in channel_event (event=3, info=0x35e9340)
     at /home/kraxel/projects/qemu/ui/spice-core.c:223
#4  0x00007f9a77a9921b in reds_channel_event (s=0x35e92c0) at reds.c:400
#5  reds_stream_free (s=0x35e92c0) at reds.c:4981
#6  0x00007f9a77aac8b0 in red_disconnect_channel 
(channel=0x7f9a24069a80) at red_worker.c:8489
#7  0x00007f9a77ab53a8 in handle_dev_input (listener=0x7f9a3211ab20, 
events=<value optimized out>)
     at red_worker.c:10062
#8  0x00007f9a77ab436d in red_worker_main (arg=<value optimized out>) at 
red_worker.c:10304
#9  0x0000003cec2077e1 in start_thread () from /lib64/libpthread.so.0
#10 0x0000003cebae68ed in clone () from /lib64/libc.so.6

IMHO spice server should handle the display channel tear-down in the 
dispatcher instead of the worker thread.  Alon?

>> Anyways, this commit fixes the problem at hand.

Not really.  channel_event() itself isn't thread-safe too, it does 
unlocked list operations which can also blow up when called from 
different threads.

A patch like the attached (warning: untested) should do as quick&dirty 
fix for stable.  But IMO we really should fix spice instead.

cheers,
   Gerd

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