Re: [Qemu-devel] [PATCH v5 3/4] monitor: more comments on lock-free fleids/funcs

[Peter Xu <peterx@redhat.com>]

On Wed, May 23, 2018 at 10:29:37AM +0200, Markus Armbruster wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Thu, May 17, 2018 at 02:46:36PM +0200, Markus Armbruster wrote:
> >> Peter Xu <peterx@redhat.com> writes:
> >> 
> >> > Add some explicit comment for both Readline and cpu_set/cpu_get helpers
> >> > that they do not need the mon_lock protection.
> >> 
> >> Appreciated!
> >> 
> >> > Signed-off-by: Peter Xu <peterx@redhat.com>
> >> > ---
> >> >  monitor.c | 5 +++--
> >> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >> >
> >> > diff --git a/monitor.c b/monitor.c
> >> > index d6c3c08932..ae5bca9d7c 100644
> >> > --- a/monitor.c
> >> > +++ b/monitor.c
> >> > @@ -207,7 +207,7 @@ struct Monitor {
> >> >      int suspend_cnt;            /* Needs to be accessed atomically */
> >> >      bool skip_flush;
> >> >      bool use_io_thr;
> >> > -    ReadLineState *rs;
> >> > +    ReadLineState *rs;   /* Only used in parser, so no lock needed. */
> >> 
> >> Pardon the ignorant question: why does "only used in parser" imply "no
> >> lock needed"?
> >
> > Since even if the monitors can be run in multiple threads now, the
> > monitor parser of a specific Monitor will still only be run in either
> > the main thread or the monitor iothread.  My fault to be unclear on
> > the comment.  Maybe this one is better:
> >
> >   It is only used in parser, and the parser of a monitor will only be
> >   run either in main thread or monitor IOThread but never both, so no
> >   lock is needed when accessing ReadLineState.
> 
> One further question, just to help me understand how this stuff works:
> what are the conditions for the parser running in the main thread, and
> what are the conditions for it running in the monitor IOThread?

For QMP parsers, the place is decided by Monitor.use_io_thr.  If set,
the parser runs in monitor IOThread; otherwise it still runs in main
thread.

For HMP parsers, they should always been run in the main thread.

After replying I just noticed that ReadLineState should only be used
by HMP, or to be more explicit, when MONITOR_USE_READLINE is set.  So
maybe the comment is not really accurate above - actually it never
runs in monitor iothread!  However the conclusion is still the same -
we don't need to protect it.

Thanks,

-- 
Peter Xu