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

[Marc-André Lureau]

Hi

On Mon, Nov 20, 2023 at 5:36 PM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> 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
>
> ZVT-bY9YOr69QTPX@redhat.com/">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
>
>

How reliably? I tried 10/10.

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