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

[Marcelo Tosatti <mtosatti@redhat.com>]

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:

diff --git a/qemu-kvm.c b/qemu-kvm.c
index 87ece3d..141c8b1 100644
--- a/qemu-kvm.c
+++ b/qemu-kvm.c
@@ -1931,7 +1931,8 @@ static int kvm_main_loop_cpu(CPUState *env)
         }
         if (run_cpu) {
             kvm_main_loop_wait(env, 0);
-            kvm_cpu_exec(env);
+            if (!is_cpu_stopped(env))
+                kvm_cpu_exec(env);
         } else {
             kvm_main_loop_wait(env, 1000);
         }