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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 02 Sep 2011 15:39:03 +0200
Gerd Hoffmann <kraxel@redhat.com> 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.

I thought my patch was at least a candidate for stable, but after this
thread I'm convinced the problem should be fixed in spice instead.

> 
> 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
>