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

[Anthony Liguori <anthony@codemonkey.ws>]

On 09/02/2011 08:39 AM, Gerd Hoffmann wrote:
> 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.

I agree.  I'm not sure I like the idea of still calling QEMU code 
without holding the mutex (even the QObject code).

Can you just use a bottom half to defer this work to the I/O thread? 
Bottom half scheduling has to be signal safe which means it will also be 
thread safe.

Regards,

Anthony Liguori

>
> cheers,
> Gerd
>