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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] monitor: Protect outbuf from concurrent access
Date: Fri, 02 Sep 2011 09:03:47 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110516 Lightning/1.0b2 Thunderbird/3.1.10

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

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
#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.

Spice should not be calling *any* QEMU code without holding the global mutex. That includes all of the QObject interactions.


Anthony Liguori


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