Re: [Qemu-devel] [RFC v4 21/27] qmp: let migrate-incoming allow out-of-band

[Peter Xu]

On Fri, Nov 24, 2017 at 01:14:53PM +0000, Dr. David Alan Gilbert wrote:
> * Peter Xu (address@hidden) wrote:
> > So it can get rid of being run on main thread.
> > 
> > Signed-off-by: Peter Xu <address@hidden>
> 
> Last time I asked if you were sure that we didn't do locking,
> and you explained that we end up just setting up a callback
> that happens in the mainloop, and this shouldn't take a lock.
> I confirmed this by:
> 
>   running with -incoming defer
>   breakpointing in hmp_migrate_incoming
>   doing   migrate_incoming tcp:0:4444
>   once I hit that breakpointing adding two more breakpoints:
>      a) qemu_mutex_lock_iothread
>      b) the end of hmp's handle_hmp_command
> 
>   and indeed it hit the end of handle_hmp_command without
> having hit the qemu_mutex_lock_iothread; so initially that
> looks ok.

I am not sure I fully understand the test above - I think it was
trying to verify the whole OOB thing running without BQL?  If so,
there are possibly two things missing:

Firstly, qemu_mutex_lock_iothread() is actually called before we call
hmp_migrate_incoming(). To verify it is simple: just break at entry of
hmp_migrate_incoming() and do "p iothread_locked", it'll be true (I
would always prefer printing that global variable to know whether
current thread is in a BQL section).

Secondly, HMP will still always take the BQL; this patch only enables
OOB for QMP command "migrate-incoming" rather than the HMP command. So
IMHO what we need to test is QMP command, rather than this HMP one.

To test that QMP command, it's still not that easy (actually awkward).
We need to first enable "OOB" when doing handshake for QMP:

  {"execute": "qmp_capabilities", "arguments": { "enable": [ "oob" ] } }

Then, we need to send the command with proper "id"/"control" field:

  { "execute": "migrate-incoming",
    "arguments": { "uri": "tcp:localhost:1234" },
    "control": { "run-oob": true },
    "id": "test-command" }

Note that here "id" field will be a must now since OOB requires that,
meanwhile the "control" field is a must too to make sure that is run
in OOB format (otherwise this command will still take BQL and run as
usual).  So if you see we do have a lot of protection to make sure we
only run OOB only if we really wanted to... otherwise we'll always run
in compatible and old way.

This time if we break at qmp_migrate_incoming() and then do "p
iothread_locked", we should see a false here.

> 
> But then I added a break on pthread_mutex_lock, and I've got
> this set caused by qemu_start_incoming_migration sending a
> MIGRATION_STATUS_SETUP event:
> 
> #1  0x0000555555ba6eba in qemu_mutex_lock (address@hidden <monitor_lock>)
>     at /home/dgilbert/git/qemu/util/qemu-thread-posix.c:65
> #2  0x00005555557f01c1 in monitor_qapi_event_queue 
> (event=QAPI_EVENT_MIGRATION, qdict=0x555557d93c00, errp=<optimized out>) at 
> /home/dgilbert/git/qemu/monitor.c:442
> 
> 440       trace_monitor_protocol_event_queue(event, qdict, evconf->rate);
> 441   
> 442       qemu_mutex_lock(&monitor_lock);
> 443   
> 
> #3  0x0000555555b92722 in qapi_event_send_migration (address@hidden, 
> errp=0x555556859320 <error_abort>) at qapi-event.c:661
> #4  0x0000555555a6159f in qemu_start_incoming_migration (uri=0x555557693f30 
> "tcp:0:4444", errp=0x7fffffffc700)
>     at /home/dgilbert/git/qemu/migration/migration.c:253
> #5  0x0000555555a641c5 in qmp_migrate_incoming (uri=0x555557693f30 
> "tcp:0:4444", errp=0x7fffffffc730)
>     at /home/dgilbert/git/qemu/migration/migration.c:1321
> 
> is there anything which protects us there?

IIUC you mean what if we e.g. page fault during taking the
monitor_lock?  IMHO it just can't happen - monitor_lock is really used
in limited places and during those critical sections there is no guest
memory access at all (which only protects the monitor logic itself
AFAICT).

Thanks,

> 
> Dave
>     
> > ---
> >  qapi/migration.json | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/qapi/migration.json b/qapi/migration.json
> > index bbc4671ded..95098072dd 100644
> > --- a/qapi/migration.json
> > +++ b/qapi/migration.json
> > @@ -1063,7 +1063,8 @@
> >  # <- { "return": {} }
> >  #
> >  ##
> > -{ 'command': 'migrate-incoming', 'data': {'uri': 'str' } }
> > +{ 'command': 'migrate-incoming', 'data': {'uri': 'str' },
> > +  'allow-oob': true }
> >  
> >  ##
> >  # @xen-save-devices-state:
> > -- 
> > 2.13.6
> > 
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK

-- 
Peter Xu