Re: [RFC 1/3] hmp: avoid the nested event loop in handle_hmp_command()

[Stefan Hajnoczi]

On Thu, Sep 07, 2023 at 01:06:39AM +0000, Dr. David Alan Gilbert wrote:
> * Stefan Hajnoczi (stefanha@redhat.com) wrote:
> > Coroutine HMP commands currently run to completion in a nested event
> > loop with the Big QEMU Lock (BQL) held. The call_rcu thread also uses
> > the BQL and cannot process work while the coroutine monitor command is
> > running. A deadlock occurs when monitor commands attempt to wait for
> > call_rcu work to finish.
> 
> I hate to think if there's anywhere else that ends up doing that
> other than the monitors.

Luckily drain_call_rcu() has few callers: just
xen_block_device_destroy() and qmp_device_add(). We only need to worry
about their call stacks.

I haven't looked at the Xen code.

> 
> But, not knowing the semantics of the rcu code, it looks kind of OK to
> me from the monitor.
> 
> (Do you ever get anything like qemu quitting from one of the other
> monitors while this coroutine hasn't been run?)

Not sure what you mean?

Stefan

> 
> Dave
> 
> > This patch refactors the HMP monitor to use the existing event loop
> > instead of creating a nested event loop. This will allow the next
> > patches to rely on draining call_rcu work.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  monitor/hmp.c | 28 +++++++++++++++-------------
> >  1 file changed, 15 insertions(+), 13 deletions(-)
> > 
> > diff --git a/monitor/hmp.c b/monitor/hmp.c
> > index 69c1b7e98a..6cff2810aa 100644
> > --- a/monitor/hmp.c
> > +++ b/monitor/hmp.c
> > @@ -1111,15 +1111,17 @@ typedef struct HandleHmpCommandCo {
> >      Monitor *mon;
> >      const HMPCommand *cmd;
> >      QDict *qdict;
> > -    bool done;
> >  } HandleHmpCommandCo;
> >  
> > -static void handle_hmp_command_co(void *opaque)
> > +static void coroutine_fn handle_hmp_command_co(void *opaque)
> >  {
> >      HandleHmpCommandCo *data = opaque;
> > +
> >      handle_hmp_command_exec(data->mon, data->cmd, data->qdict);
> >      monitor_set_cur(qemu_coroutine_self(), NULL);
> > -    data->done = true;
> > +    qobject_unref(data->qdict);
> > +    monitor_resume(data->mon);
> > +    g_free(data);
> >  }
> >  
> >  void handle_hmp_command(MonitorHMP *mon, const char *cmdline)
> > @@ -1157,20 +1159,20 @@ void handle_hmp_command(MonitorHMP *mon, const char 
> > *cmdline)
> >          Monitor *old_mon = monitor_set_cur(qemu_coroutine_self(), 
> > &mon->common);
> >          handle_hmp_command_exec(&mon->common, cmd, qdict);
> >          monitor_set_cur(qemu_coroutine_self(), old_mon);
> > +        qobject_unref(qdict);
> >      } else {
> > -        HandleHmpCommandCo data = {
> > -            .mon = &mon->common,
> > -            .cmd = cmd,
> > -            .qdict = qdict,
> > -            .done = false,
> > -        };
> > -        Coroutine *co = qemu_coroutine_create(handle_hmp_command_co, 
> > &data);
> > +        HandleHmpCommandCo *data; /* freed by handle_hmp_command_co() */
> > +
> > +        data = g_new(HandleHmpCommandCo, 1);
> > +        data->mon = &mon->common;
> > +        data->cmd = cmd;
> > +        data->qdict = qdict; /* freed by handle_hmp_command_co() */
> > +
> > +        Coroutine *co = qemu_coroutine_create(handle_hmp_command_co, data);
> > +        monitor_suspend(&mon->common); /* resumed by 
> > handle_hmp_command_co() */
> >          monitor_set_cur(co, &mon->common);
> >          aio_co_enter(qemu_get_aio_context(), co);
> > -        AIO_WAIT_WHILE_UNLOCKED(NULL, !data.done);
> >      }
> > -
> > -    qobject_unref(qdict);
> >  }
> >  
> >  static void cmd_completion(MonitorHMP *mon, const char *name, const char 
> > *list)
> > -- 
> > 2.41.0
> > 
> > 
> -- 
>  -----Open up your eyes, open up your mind, open up your code -------   
> / Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
> \        dave @ treblig.org |                               | In Hex /
>  \ _________________________|_____ http://www.treblig.org   |_______/
>

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