Re: [Qemu-devel] Abort in monitor_puts.

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 25 Mar 2013 08:42:57 +0100
Gerd Hoffmann <kraxel@redhat.com> wrote:

> On 03/22/13 22:39, Luiz Capitulino wrote:
> > On Fri, 22 Mar 2013 16:50:39 -0400
> > Luiz Capitulino <lcapitulino@redhat.com> wrote:
> > 
> >> On Fri, 22 Mar 2013 10:17:58 +0100
> >> KONRAD Frédéric <fred.konrad@greensocs.com> wrote:
> >>
> >>> Hi,
> >>>
> >>> Seems there is an issue with the current git (found by toddf on IRC).
> >>>
> >>> To reproduce:
> >>>
> >>> ./qemu-system-x86_64 --monitor stdio --nographic
> >>>
> >>> and put "?" it should abort.
> >>>
> >>> Here is the backtrace:
> >>>
> >>> #0  0x00007f77cd347935 in raise () from /lib64/libc.so.6
> >>> #1  0x00007f77cd3490e8 in abort () from /lib64/libc.so.6
> >>> #2  0x00007f77cd3406a2 in __assert_fail_base () from /lib64/libc.so.6
> >>> #3  0x00007f77cd340752 in __assert_fail () from /lib64/libc.so.6
> >>> #4  0x00007f77d1c1f226 in monitor_puts (mon=<optimized out>,
> >>>      str=<optimized out>) at 
> >>
> >> Yes, it's easy to reproduce. Bisect says:
> >>
> >> f628926bb423fa8a7e0b114511400ea9df38b76a is the first bad commit
> >> commit f628926bb423fa8a7e0b114511400ea9df38b76a
> >> Author: Gerd Hoffmann <kraxel@redhat.com>
> >> Date:   Tue Mar 19 10:57:56 2013 +0100
> >>
> >>     fix monitor
> >>     
> >>     chardev flow control broke monitor, fix it by adding watch support.
> >>     
> >>     Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
> >>
> >> My impression is that monitor_puts() in being called in parallel.
> > 
> > Not all.
> > 
> > What's happening is that qemu_chr_fe_write() is returning < 0,
> > mon->outbuf_index is not reset and is full, this causes the assert in
> > monitor_puts() to trig.
> > 
> > The previous version of monitor_flush() ignores errors, and everything
> > works, so doing the same thing here fixes the problem :)
> 
> No, ignoring errors breaks qmp because the output isn't valid json any
> more when you cut off something ...

What you mean "when you cut off"? When the other side disconnects? Do we care?

> > For some reason I'm unable to see what the error code is. Gerd, do you think
> > the patch below is reasonable? If it's not, how should we handle errors here?
> 
> No, it's not.
> 
> Ignoring the error for errno = EAGAIN breaks flow control.
> 
> Ignoring the error for errno != EAGAIN (and maybe logging a debug
> message) would be ok, but I suspect it's actually EAGAIN you get here.
> 
> Just go for a larger buffer?

That's simple, but it's not a real fix. We hit that problem because
the help output is a large one. I'd guess that this is easily reproduced
with something like QIDL, which (iirc) generates long json output on QMP.

Looks like we need a dynamic buffer there.

Other ideas?