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

[Peter Xu]

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

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)?

Regards,

-- 
Peter Xu