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Re: [libvirt] segment fault from libvirtmod



On Mon, Feb 06, 2012 at 03:59:19PM +0100, Michal Privoznik wrote:
> On 06.02.2012 10:08, Guannan Ren wrote:
> > 
> > 
> >     Hi,
> > 
> >         The Makefile.am in python forget to add probes.o if WITH_DTRACE
> >         but after I added it and tried to connect, using
> > libvirt.open("qemu:///system")
> >         in python , it reported: "segment fault"
> >         I tried to figure out, but failed. If anyone can help this, thanks.
> > 
> >         The following is the backtrace:
> > 
> >        #0  0x0000003c62a810a4 in free () from /lib64/libc.so.6
> >        #1  0x00007ffff1836f29 in virFree (ptrptr=0x6a6a28) at
> > util/memory.c:310
> >        #2  0x00007ffff1849692 in virResetError (err=0x6a6a20) at
> > util/virterror.c:387
> >        #3  0x00007ffff13e2761 in do_open (name=0x7ffff7ed6444
> > "qemu:///system", auth=0x0, flags=0) at libvirt.c:1093
> >        #4  0x00007ffff13e4d44 in virConnectOpen (name=0x7ffff7ed6444
> > "qemu:///system") at libvirt.c:1350
> >        #5  0x00007ffff1824879 in libvirt_virConnectOpen (self=<optimized
> > out>, args=<optimized out>) at libvirt.c:3637
> >        #6  0x0000003c652dffbb in PyEval_EvalFrameEx () from
> > /usr/lib64/libpython2.7.so.1.0
> >        #7  0x0000003c652e0580 in PyEval_EvalFrameEx () from
> > /usr/lib64/libpython2.7.so.1.0
> >        #8  0x0000003c652e15a5 in PyEval_EvalCodeEx () from
> > /usr/lib64/libpython2.7.so.1.0
> >        #9  0x0000003c652e16d2 in PyEval_EvalCode () from
> > /usr/lib64/libpython2.7.so.1.0
> >        #10 0x0000003c652fb9ec in ?? () from /usr/lib64/libpython2.7.so.1.0
> >        #11 0x0000003c652fc7f0 in PyRun_FileExFlags () from
> > /usr/lib64/libpython2.7.so.1.0
> >        #12 0x0000003c652fd26f in PyRun_SimpleFileExFlags () from
> > /usr/lib64/libpython2.7.so.1.0
> >        #13 0x0000003c6530e745 in Py_Main () from
> > /usr/lib64/libpython2.7.so.1.0
> >        #14 0x0000003c62a2169d in __libc_start_main () from /lib64/libc.so.6
> >        #15 0x0000000000400651 in _start ()
> > 
> >      Guannan Ren
> > 
> 
> 
> Running git bisect showed it was caused by
> c700613b8d463212d142c97108b7a2352e23e559. However, I think it only
> exposed the design problem we are facing here.
> 
> IMHO problem is when an application tries to call virConnectOpen() (or
> virConnectOpenAuth()) from two concurrent threads. This is what will happen:
> 
> 1st thread:
>   call virConnectOpen();
>   global variable @initialized == zero; thus virInitialize() is called,
> error object is set up (via virErrorInitialize()) which is thread local
> variable. However, @initialized is set to 1 in the first place.
> 
> 2nd thread:
>   call virConnectOpen() as well;
>   @initialized is already set, therefore no thread local variable is set
> and any error report will fail.
> 
> Even if I am right, I am not sure how to fix this.

In a mulithreaded application, you should be calling virInitialize()
explicitly, instead of relying on lazy init.  IMHO, the python module
should simply be calling virInitialize at time of import.

Regards,
Daniel
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