[Qemu-devel] Re: segmentation fault in qemu-kvm-0.14.0

[Peter Lieven]

Am 09.03.2011 um 08:37 schrieb Jan Kiszka:

> On 2011-03-08 23:53, Peter Lieven wrote:
>> Hi,
>> 
>> during testing of qemu-kvm-0.14.0 i can reproduce the following segfault. i 
>> have seen similar crash already in 0.13.0, but had no time to debug.
>> my guess is that this segfault is related to the threaded vnc server which 
>> was introduced in qemu 0.13.0. the bug is only triggerable if a vnc
>> client is attached. it might also be connected to a resolution change in the 
>> guest. i have a backtrace attached. the debugger is still running if someone
>> needs more output
>> 
> 
> ...
> 
>> Thread 1 (Thread 0x7ffff7ff0700 (LWP 29038)):
>> #0  0x0000000000000000 in ?? ()
>> No symbol table info available.
>> #1  0x000000000041d669 in main_loop_wait (nonblocking=0)
>>    at /usr/src/qemu-kvm-0.14.0/vl.c:1388
> 
> So we are calling a IOHandlerRecord::fd_write handler that is NULL.
> Looking at qemu_set_fd_handler2, this may happen if that function is
> called for an existing io-handler entry with non-NULL write handler,
> passing a NULL write and a non-NULL read handler. And all this without
> the global mutex held.
> 
> And there are actually calls in vnc_client_write_plain and
> vnc_client_write_locked (in contrast to vnc_write) that may generate
> this pattern. It's probably worth validating that the iothread lock is
> always held when qemu_set_fd_handler2 is invoked to confirm this race
> theory, adding something like
> 
> assert(pthread_mutex_trylock(&qemu_mutex) != 0);
> (that's for qemu-kvm only)

qemu_mutex doesn't exists (anymore). i can only find qemu_global_mutex and 
qemu_fair_mutex.

i try with qemu_global_mutex now. if thats not correct please tell.

peter

> 
> BTW, qemu with just --enable-vnc-thread, ie. without io-thread support,
> should always run into this race as it then definitely lacks a global mutex.
> 
> Jan
>