* Stefan Hajnoczi (stefa...@gmail.com) wrote:
> On Thu, 7 Sept 2023 at 10:07, Dr. David Alan Gilbert <d...@treblig.org> wrote:
> >
> > * Stefan Hajnoczi (stefa...@redhat.com) wrote:
> > > On Thu, Sep 07, 2023 at 01:06:39AM +0000, Dr. David Alan Gilbert wrote:
> > > > * Stefan Hajnoczi (stefa...@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?
> >
> > Imagine that just after you create your coroutine, a vCPU does a
> > shutdown and qemu is configured to quit, or on another monitor someone
> > does a quit; does your coroutine get executed or not?
>
> I think the answer is that it depends.
>
> A coroutine can run for a while and then yield while waiting for a
> timer, BH, fd handler, etc. If the coroutine has yielded then I think
> QEMU could terminate.
>
> The behavior of entering a coroutine for the first time depends on the
> API that is used (e.g. qemu_coroutine_enter()/aio_co_enter()/etc).
> qemu_coroutine_enter() is immediate but aio_co_enter() contains
> indirect code paths like scheduling a BH.
>
> To summarize: ¯\_(ツ)_/¯
That does mean you leave your g_new'd data and qdict allocated at
exit - meh
I'm not sure if it means you're making any other assumptions about what
happens if the coroutine gets run during the exit path; although I guess
there are plenty of other cases like that.
Dave
> Stefan
>
> >
> > Dave
> >
> > > 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 <stefa...@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 |_______/
> > > >
> >
> >
> > --
> > -----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 |_______/
> >
--
-----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 |_______/
