Re: [Qemu-devel] qemu-nbd vs 'simple' trace backend vs iotest 147

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 11 Jul 2018 14:06:17 +0100
Stefan Hajnoczi <stefanha@gmail.com> wrote:

> On Mon, Jul 09, 2018 at 03:45:49PM +0200, Cornelia Huck wrote:
> > Hi,
> > 
> > I recently noticed that iotest 147 was hanging on my laptop, but worked
> > fine on my s390x LPAR. Turned out that the architecture was a red
> > herring; on both platforms, things fail with the 'simple' trace backend
> > and work with e.g. the 'log' trace backend. Some details on the
> > failures with the 'simple' backend:
> > 
> > - The first run of 147 passes. However, there are two processes hanging
> >   around, one using a unix socket and one using an inet socket:
> > 
> > cohuck   22912  0.0  0.0 156580  3836 ?        Ss   14:32   0:00 /home/cohuck/git/qemu/build/tests/qemu-iotests/../../qemu-nbd --fork -f qcow2 /home/cohuck/git/qemu/build/tests/qemu-iotests/scratch/test.img -p 10811
> > cohuck   22925  0.0  0.0 156580  3840 ?        Ss   14:32   0:00 /home/cohuck/git/qemu/build/tests/qemu-iotests/../../qemu-nbd --fork -f qcow2 /home/cohuck/git/qemu/build/tests/qemu-iotests/scratch/test.img -k /home/cohuck/git/qemu/build/tests/qemu-iotests/scratch/nbd.socket
> > 
> > Attaching a gdb shows that we seem to be waiting on flushing:
> > 
> > (gdb) bt
> > #0  0x00007f461c078b99 in syscall () from /lib64/libc.so.6
> > #1  0x00007f461d13650f in g_cond_wait () from /lib64/libglib-2.0.so.0
> > #2  0x0000560cf3a1caf2 in flush_trace_file (wait=255)
> >     at /home/cohuck/git/qemu/trace/simple.c:139
> > #3  st_flush_trace_buffer () at /home/cohuck/git/qemu/trace/simple.c:374
> > #4  0x00007f461bfc01d8 in __run_exit_handlers () from /lib64/libc.so.6
> > #5  0x00007f461bfc022a in exit () from /lib64/libc.so.6
> > #6  0x0000560cf392eb7e in main (argc=<optimized out>, argv=<optimized out>)
> >     at /home/cohuck/git/qemu/qemu-nbd.c:1076
> > 
> > (for both processes)  
> 
> Please also print backtraces for the other threads:
> 
>   (gdb) thread apply all bt
> 
> There should be another thread in writeout_thread() so I'm surprised
> that flush_trace_file() is getting stuck in g_cond_wait().

I'll re-run to check, but there was only one thread in the process in
question.