Re: [Qemu-devel] [RFC v5 09/26] monitor: create monitor dedicate iothread

[Peter Xu <peterx@redhat.com>]

On Sat, Dec 16, 2017 at 12:42:00PM +0800, Peter Xu wrote:
> On Fri, Dec 15, 2017 at 01:21:42PM +0000, Stefan Hajnoczi wrote:
> > On Fri, Dec 15, 2017 at 04:31:08PM +0800, Peter Xu wrote:
> > > On Wed, Dec 13, 2017 at 04:20:22PM +0000, Stefan Hajnoczi wrote:
> > > > On Tue, Dec 05, 2017 at 01:51:43PM +0800, Peter Xu wrote:
> > > > > @@ -208,6 +209,12 @@ struct Monitor {
> > > > >      QTAILQ_ENTRY(Monitor) entry;
> > > > >  };
> > > > >  
> > > > > +struct MonitorGlobal {
> > > > > +    IOThread *mon_iothread;
> > > > > +};
> > > > > +
> > > > > +static struct MonitorGlobal mon_global;
> > > > 
> > > > structs can be anonymous.  That avoids the QEMU coding style violation
> > > > (structs must be typedefed):
> > > > 
> > > >   static struct {
> > > >       IOThread *mon_iothread;
> > > >   } mon_global;
> > > 
> > > Will fix this up.  Thanks.
> > > 
> > > > 
> > > > In general global variables are usually top-level variables in QEMU.
> > > > I'm not sure why wrapping globals in a struct is useful.
> > > 
> > > Because I see too many global variables for monitor code, and from
> > > this patch I wanted to start moving them altogether into this global
> > > struct.  I didn't really do that in current series because it's more
> > > like a clean up, but if you see future patches,
> > 
> > You cannot expect reviewers to jump around a 26 patch series to check
> > for possible future changes.  Each patch must be self-contained and the
> > changes need to be justified.
> 
> Noted.
> 
> > 
> > > I can add a comment in the commit message, like: "Let's start to
> > > create a struct to keep monitor global variables together".  Would
> > > that help?
> > 
> > It's better to add a comment in the code:
> > 
> > /* Add monitor global variables to this struct */
> > 
> > so that other people modifying the code know what this is about and can
> > participate.
> 
> Will do.
> 
> > 
> > Other people might not want to do it since it leads to repetitive and
> > long names like mon_global.mon_iothread.  Or they might just not see the
> > struct when defining a global.
> > 
> > The chance of the struct being used consistently is low and therefore I
> > wouldn't do it.
> > 
> > > > 
> > > > > @@ -4117,6 +4136,16 @@ void monitor_cleanup(void)
> > > > >  {
> > > > >      Monitor *mon, *next;
> > > > >  
> > > > > +    /*
> > > > > +     * We need to explicitly stop the iothread (but not destroy it),
> > > > > +     * cleanup the monitor resources, then destroy the iothread.  See
> > > > > +     * again on the glib bug mentioned in 2b316774f6 for a reason.
> > > > > +     *
> > > > > +     * TODO: the bug is fixed in glib 2.28, so we can remove this hack
> > > > > +     * as long as we won't support glib versions older than it.
> > > > > +     */
> > > > 
> > > > I find this comment confusing.  There is no GSource .finalize() in
> > > > monitor.c so why does monitor_cleanup() need to work around the bug?
> > > > 
> > > > I see that monitor_data_destroy() is not thread-safe so the IOThread
> > > > must be stopped first.  That is unrelated to glib.
> > > 
> > > Yeah actually that's a suggestion by Dave and Dan in previous review
> > > comments which makes sense to me:
> > > 
> > >   http://lists.gnu.org/archive/html/qemu-devel/2017-11/msg04344.html
> > > 
> > > I'm fine with either way: keep it as it is,
> > 
> > The meaning of the comment is unclear to me and you haven't been able to
> > explain it.  Therefore, merging this comment isn't justified.
> 
> (Please see below)
> 
> > 
> > > or instead saying
> > > "monitor_data_destroy() is not thread-safe" (which finally will still
> > > root cause to that glib bug).
> > 
> > This is incorrect.  The problem is that the IOThread may run chardev
> > handler functions while the main loop thread invokes
> > monitor_data_destroy().  There is nothing protecting the chardev itself
> > (it's not thread-safe!) nor the monitor state that is being freed, so a
> > running chardev handler function could crash.
> 
> It's only about a single line of comment, but since we are at this, I
> think it would still be good to discuss it in case I was wrong.
> 
> Firstly, I agree that chardevs are not thread-safe.  But IMHO monitors
> are thread-safe.  There is the big monitor_lock to protect.  There can
> be bug though, but generally speaking that lock should be doing the
> thread safety work.
> 
> Next, basically when destroying the monitors logically we should never
> touch the chardev if without that glib bug.  Or say, if without the
> bug we should not call qemu_chr_fe_deinit() in monitor_data_destroy().

Ouch. :-/

I was meaning remove_fd_in_watch() in qemu_chr_fe_set_handlers().  Of
course it needs to touch chardev to unregister the stuff. :-)

And I think you are right.  Let me remove the comment.  The
iothread_stop() needs to be there always, even without that bug.

Sorry for the noise.

-- 
Peter Xu