Re: [PATCH 2/3] chardev: report blocked write to chardev backend

["Nicholas Piggin" <npiggin@gmail.com>]

On Mon Nov 20, 2023 at 10:06 PM AEST, Marc-André Lureau wrote:
> Hi
>
> On Thu, Nov 16, 2023 at 3:54 PM Nicholas Piggin <npiggin@gmail.com> wrote:
> >
> > If a chardev socket is not read, it will eventually fill and QEMU
> > can block attempting to write to it. A difficult bug in avocado
> > tests where the console socket was not being read from caused this
> > hang.
> >
> > warn if a chardev write is blocked for 100ms.
> >
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > ---
> > This is not necessary for the fix but it does trigger in the
> > failing avocado test without the previous patch applied. Maybe
> > it would be helpful?
> >
> > Thanks,
> > Nick
> >
> >  chardev/char.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/chardev/char.c b/chardev/char.c
> > index 996a024c7a..7c375e3cc4 100644
> > --- a/chardev/char.c
> > +++ b/chardev/char.c
> > @@ -114,6 +114,8 @@ static int qemu_chr_write_buffer(Chardev *s,
> >  {
> >      ChardevClass *cc = CHARDEV_GET_CLASS(s);
> >      int res = 0;
> > +    int nr_retries = 0;
> > +
> >      *offset = 0;
> >
> >      qemu_mutex_lock(&s->chr_write_lock);
> > @@ -126,6 +128,10 @@ static int qemu_chr_write_buffer(Chardev *s,
> >              } else {
> >                  g_usleep(100);
> >              }
> > +            if (++nr_retries == 1000) { /* 100ms */
> > +                warn_report("Chardev '%s' write blocked for > 100ms, "
> > +                            "socket buffer full?", s->label);
> > +            }
>
> That shouldn't happen, the frontend should poll and only write when it
> can. What is the qemu command being used here?

You can follow it through the thread here

https://lore.kernel.org/qemu-devel/ZVT-bY9YOr69QTPX@redhat.com/

In short, a console device is attached to a socket pair and nothing
ever reads from it. It eventually fills, and writing to it fails
indefinitely here.

It can be reproduced with:

make check-avocado
AVOCADO_TESTS=tests/avocado/reverse_debugging.py:test_ppc64_pseries


> I think this change can be worth for debugging though.
>
> Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

Thanks,
Nick