Re: [Qemu-devel] [PATCH rebased 2/2] monitor: delay monitor iothread creation

[Peter Xu]

On Thu, Sep 27, 2018 at 02:35:07PM +0200, Markus Armbruster wrote:
> Peter Xu <address@hidden> writes:
> 
> > On Thu, Sep 27, 2018 at 10:46:34AM +0200, Markus Armbruster wrote:
> >> Peter Xu <address@hidden> writes:
> >> 
> >> > On Tue, Sep 25, 2018 at 01:09:57PM +0200, Wolfgang Bumiller wrote:
> >> >> 
> >> >> > On September 25, 2018 at 12:31 PM Peter Xu <address@hidden> wrote:
> >> >> > 
> >> >> > 
> >> >> > On Tue, Sep 25, 2018 at 10:15:07AM +0200, Wolfgang Bumiller wrote:
> >> >> > > Commit d32749deb615 moved the call to monitor_init_globals()
> >> >> > > to before os_daemonize(), making it an unsuitable place to
> >> >> > > spawn the monitor iothread as it won't be inherited over the
> >> >> > > fork() in os_daemonize().
> >> >> > > 
> >> >> > > We now spawn the thread the first time we instantiate a
> >> >> > > monitor which actually has use_io_thread == true.
> >> >> > > Instantiation of monitors happens only after os_daemonize().
> >> >> > > We still need to create the qmp_dispatcher_bh when not using
> >> >> > > iothreads, so this now still happens in
> >> >> > > monitor_init_globals().
> >> >> > > 
> >> >> > > Signed-off-by: Wolfgang Bumiller <address@hidden>
> >> >> > > Fixes: d32749deb615 ("monitor: move init global earlier")
> >> >> > 
> >> >> > Reviewed-by: Peter Xu <address@hidden>
> >> >> > Tested-by: Peter Xu <address@hidden>
> >> >> > 
> >> >> > Though note that after this patch monitor_data_init() is not thread
> >> >> > safe any more (while it was), so we may need to be careful...
> >> >> 
> >> >> Is there a way to create monitors concurrently? If so, we could
> >> >> add a mutex initialized in monitor_globals_init().
> >> >
> >> > qmp_human_monitor_command() creates monitors dynamically, though not
> >> > concurrently since currently it's only possible to happen on the main
> >> > thread (and it's always setting use_io_thread to false now).  So we
> >> > should be safe now.
> >> 
> >> Until recently, monitor code ran entirely in the main loop.
> >> Undesirable, because it lets monitors hog the main loop.
> >> 
> >> Moving stuff out of the main loop is non-trivial, because it may break
> >> unstated assumptions.
> >> 
> >> Peter's OOB work moved the monitor core from the main loop into
> >> @mon_iothread.
> >> 
> >> Moving commands is harder: you have to audit each command for
> >> assumptions that no longer hold.  A common one is of course "thread
> >> safety is not an issue".  Peter's OOB work provides for OOB command
> >> execution in @mon_iothread.
> >> 
> >> As long as monitors get created dynamically only in monitor commands,
> >> the lack of synchronization around the creation of @mon_iothread is an
> >> instance of "monitor commands assume they're running in the main loop".
> >> 
> >> >> Another way would be to only defer to after os_daemonize() but still
> >> >> stick to spawning the thread unconditionally. (Iow. call
> >> >> monitor_iothread_init() after os_daemonize() from vl.c.)
> >
> > [1]
> >
> >> >
> >> > Yeah I think that should work too (and seems good).  I'll see how
> >> > Markus think.
> >> 
> >> My first thought was:
> >> 
> >>     diff --git a/monitor.c b/monitor.c
> >>     index 44d068b106..50f7d1230f 100644
> >>     --- a/monitor.c
> >>     +++ b/monitor.c
> >>     @@ -712,9 +712,7 @@ static void monitor_iothread_init(void);
> >>      static void monitor_data_init(Monitor *mon, bool skip_flush,
> >>                                    bool use_io_thread)
> >>      {
> >>     -    if (use_io_thread && !mon_iothread) {
> >>     -        monitor_iothread_init();
> >>     -    }
> >>     +    assert(!use_io_thread || mon_iothread);
> >>          memset(mon, 0, sizeof(Monitor));
> >>          qemu_mutex_init(&mon->mon_lock);
> >>          qemu_mutex_init(&mon->qmp.qmp_queue_lock);
> >>     @@ -4555,6 +4553,9 @@ void monitor_init(Chardev *chr, int flags)
> >>                  error_report("Monitor out-of-band is only supported by 
> >> QMP");
> >>                  exit(1);
> >>              }
> >>     +        if (!mon_iothread) {
> >>     +            monitor_iothread_init();
> >>     +        }
> >>          }
> >> 
> >>          monitor_data_init(mon, false, use_oob);
> >> 
> >> This limits monitor_data_init() to initialization of *mon.  I like that.
> >> 
> >> It also makes it obvious that qmp_human_monitor_command() can't mess
> >> with @mon_iothread.  Sadly, that doesn't really buy us anything, since
> >> the other callers of monitor_init() can still mess with it.  These are:
> >> 
> >> * gdbserver_start()
> >> 
> >>   CLI option -gdb, HMP command gdbserver, linux user CLI option -g and
> >>   environment variable QEMU_GDB
> >> 
> >> * mon_init_func()
> >> 
> >>   CLI option -mon and its convenience buddies -monitor, -qmp,
> >>   -qmp-pretty
> >> 
> >> * qemu_chr_new_noreplay()
> >> 
> >>   gdbserver_start() again, and qemu_chr_new(), which is called all over
> >>   the place.
> >> 
> >> These should all run in the main loop (anything else would be a bug).
> >> They (more or less) obviously do, except for qemu_chr_new(), where we
> >> aren't sure.
> >> 
> >> Please see
> >> Subject: Re: [PATCH] monitor: avoid potential dead-lock when cleaning up
> >> Message-ID: <address@hidden>
> >> https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg03821.html
> >> 
> >> The conclusion reached there was "I'm afraid we need to rethink the set
> >> of locks protecting shared monitor state" and "probably change a bit
> >> monitor/chardev creation to be under tighter control..."
> >
> > Yeah that would be nice...
> >
> >> 
> >> Should we put @mon_iothread under @monitor_lock?
> >
> > IMHO we can when we create the thread.  I guess we don't need that
> > lock when reading @mon_iothread, after all it's a very special
> > variable in that:
> >
> >  - it is only set once, or never
> >
> >  - when reading @mon_iothread only, we must have it set or it should
> >    be a programming error, so it's more like an assert(mon_iothread)
> >    not a contention
> >
> >> 
> >> Could we accept this patch without doing that, on the theory that it
> >> doesn't make things worse than they already are?
> >
> > If this bothers us that much, how about we just choose the option that
> > Wolfgang offered at [1] above to create the iothread after daemonize
> > (so we pick that out from monitor_global_init)?
> 
> I'd prefer this patch's approach, because it keeps the interface
> simpler.
> 
> I can accept this patch as is, or with my incremental patch squashed
> in.  A comment explaining monitor_init() expects to run in the main
> thread would be nice.
> 
> I'd also accept a patch that wraps
> 
>         if (!mon_iothread) {
>             monitor_iothread_init();
>         }
> 
> in a critical section.  Using @monitor_lock is fine.  A new lock feels
> unnecessarily fine-grained.  If using @monitor_lock, move the definition
> of @mon_iothread next to @monitor_lock, and update the comment there.

Looks ok at least to me!  Thanks,

-- 
Peter Xu