[libvirt] Libvirt segfault in qemuMonitorSend() with multi-threaded API use

Adam Litke agl at us.ibm.com
Thu Mar 4 20:22:35 UTC 2010

I have a multi-threaded Python program that shares a single libvirt
connection object among several threads (one thread per active domain on
the system plus a management thread).  On a heavily loaded host with 8
running domains I am getting a consistent libvirtd segfault in the qemu
monitor handling code.  This happens with libvirt-0.7.6 and git.

Mar  4 12:23:13 bc1cn7-mgmt kernel: [ 3947.836151] libvirtd[7716]:
segfault at 24 ip 000000000045de5c sp 00007fe5aa7d2b20 error 4 in

Using addr2line, this translates to: libvirt/src/qemu/qemu_monitor.c:698

Which is in qemuMonitorSend():

--> while (!mon->msg->finished) { 
        if (virCondWait(&mon->notify, &mon->lock) < 0)
            goto cleanup;

It seems that mon->msg is being reset to NULL in the middle of this loop
execution.  I suspect that is because qemuMonitorSend() is not reentrant
and multiple threads in my program are racing here.  I would guess the
'mon->msg = NULL;' on line 707 causes the NULL that trips up the other

I presume the Monitor interface has some locking protection around it to
ensure that only one thread can use it at a time?

Is there an easy way to fix this?  I am not familiar with the measures
employed to make libvirt thread-safe.  Thanks!


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