[Qemu-devel] Re: qemu-kvm: sigsegv at exit

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 26, 2009 at 04:43:11PM -0200, Marcelo Tosatti wrote:
> > On Thu, Oct 22, 2009 at 06:57:27PM -0200, Marcelo Tosatti wrote:
> > > On Thu, Oct 22, 2009 at 02:00:15PM +0200, Michael S. Tsirkin wrote:
> > > > Hi!
> > > > I'm sometimes getting segfaults when I kill qemu.
> > > > This time I caught it when qemu was under gdb:
> > > > 
> > > > 
> > > > Program received signal SIGSEGV, Segmentation fault.
> > > > [Switching to Thread 0x411d0940 (LWP 14446)]
> > > > 0x000000000040afb4 in qemu_mod_timer (ts=0x19f0fd0, expire_time=62275467335)
> > > >     at /home/mst/scm/qemu-kvm/vl.c:1009
> > > > 1009            if ((alarm_timer->flags & ALARM_FLAG_EXPIRED) == 0) {
> > > > (gdb) l
> > > > 1004        ts->next = *pt;
> > > > 1005        *pt = ts;
> > > > 1006
> > > > 1007        /* Rearm if necessary  */
> > > > 1008        if (pt == &active_timers[ts->clock->type]) {
> > > > 1009            if ((alarm_timer->flags & ALARM_FLAG_EXPIRED) == 0) {
> > > > 1010                qemu_rearm_alarm_timer(alarm_timer);
> > > > 1011            }
> > > > 1012            /* Interrupt execution to force deadline recalculation.  */
> > > > 1013            if (use_icount)
> > > > (gdb) p alarm_timer
> > > > $1 = (struct qemu_alarm_timer *) 0x0
> > > > (gdb) where
> > > > #0  0x000000000040afb4 in qemu_mod_timer (ts=0x19f0fd0, expire_time=62275467335)
> > > >     at /home/mst/scm/qemu-kvm/vl.c:1009
> > > > #1  0x000000000041aadf in virtio_net_handle_tx (vdev=<value optimized out>, vq=0x19f5af0)
> > > >     at /home/mst/scm/qemu-kvm/hw/virtio-net.c:696
> > > > #2  0x0000000000421669 in kvm_run (vcpu=0x19d46a0, env=0x19c2250) at /home/mst/scm/qemu-kvm/qemu-kvm.c:797
> > > > #3  0x00000000004216d6 in kvm_cpu_exec (env=0x83d0f8) at /home/mst/scm/qemu-kvm/qemu-kvm.c:1714
> > > > #4  0x0000000000422981 in ap_main_loop (_env=<value optimized out>) at /home/mst/scm/qemu-kvm/qemu-kvm.c:1969
> > > > #5  0x000000377dc06367 in start_thread () from /lib64/libpthread.so.0
> > > > #6  0x000000377d0d30ad in clone () from /lib64/libc.so.6
> > > > (gdb)
> > > > 
> > > > So this probably means that we have already run quit_timers:
> > > > 
> > > > static void quit_timers(void)
> > > > {
> > > >     alarm_timer->stop(alarm_timer);
> > > >     alarm_timer = NULL;
> > > > }
> > > > 
> > > > but kvm vcpu thread is still running.
> > > > 
> > > > 
> > > > Not sure what the right fix is here: should we stop
> > > > kvm after main loop has exited?
> > > 
> > > kvm_main_loop_wait(env, 0) can process the stop request (signalling
> > > iothread that vcpu is stopped, so its OK to exit) and continue to
> > > kvm_cpu_exec.
> > > 
> > > Can you please try this:
> > 
> > I applied this, and have not yet see any segfaults at exit.
> > Not sure whether this is means anything as the crash is not
> > 100% reproducable. Push it out to Anthony and we'll see, long term?
> > Based on the knowledge of how to fix this,
> > how would you go about reproducing it?
> 
> Add code to trigger the race manually,

If you like, send a patch adding such code, I will test.

> but i'm pretty sure thats it.
> 
> Thanks for testing.