Re: [Qemu-devel] [PATCH v6 15/19] migration: Create thread infrastructure for multifd recv side

[Daniel P. Berrange]

On Wed, Sep 13, 2017 at 01:26:07PM +0200, Juan Quintela wrote:
> "Daniel P. Berrange" <address@hidden> wrote:
> > On Tue, Aug 08, 2017 at 06:26:25PM +0200, Juan Quintela wrote:
> >> We make the locking and the transfer of information specific, even if we
> >> are still receiving things through the main thread.
> >> 
> >> Signed-off-by: Juan Quintela <address@hidden>
> >> 
> >> --
> >> 
> >> We split when we create the main channel and where we start the main
> >> migration thread, so we wait for the creation of the other threads.
> >> 
> >> Use multifd_clear_group().
> >> ---
> >>  migration/migration.c |  7 ++++---
> >>  migration/migration.h |  1 +
> >>  migration/ram.c       | 55 
> >> +++++++++++++++++++++++++++++++++++++++++++++++----
> >>  migration/socket.c    |  2 +-
> >>  4 files changed, 57 insertions(+), 8 deletions(-)
> >
> >
> >> diff --git a/migration/socket.c b/migration/socket.c
> >> index 5dd6f42..3af9f7c 100644
> >> --- a/migration/socket.c
> >> +++ b/migration/socket.c
> >> @@ -183,12 +183,12 @@ static gboolean
> >> socket_accept_incoming_migration(QIOChannel *ioc,
> >>  
> >>      qio_channel_set_name(QIO_CHANNEL(sioc), "migration-socket-incoming");
> >>      migration_channel_process_incoming(QIO_CHANNEL(sioc));
> >> -    object_unref(OBJECT(sioc));
> >
> > AFAICT, migration_channel_process_incoming() acquires its own reference
> > on 'sioc', so removing this object_unref means the code is now leaking a
> > reference
> 
> Nack.
> 
> I did it and ended with a segmentation fault because reference count was
> bad.
> 
> (qemu) 
> Thread 6 "multifdrecv_0" received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7fff9f9ff700 (LWP 10065)]
> 0x00005555559c12f8 in type_is_ancestor (type=0xb60f18247c894860, 
>     target_type=0x555556531b30) at /mnt/kvm/qemu/cleanup/qom/object.c:217
> 217       while (type) {
> (gdb) bt
> #0  0x00005555559c12f8 in type_is_ancestor (type=0xb60f18247c894860, 
> target_type=0x555556531b30) at /mnt/kvm/qemu/cleanup/qom/object.c:217
> #1  0x00005555559c1dbb in object_class_dynamic_cast (address@hidden 
> <main_arena+520>, address@hidden "qio-channel")
>     at /mnt/kvm/qemu/cleanup/qom/object.c:691
> #2  0x00005555559c1ed2 in object_class_dynamic_cast_assert 
> (class=0x7ffff2cb3ce8 <main_arena+520>, address@hidden "qio-channel", 
> address@hidden "/mnt/kvm/qemu/cleanup/io/channel.c", address@hidden, 
> address@hidden <__func__.22671> "qio_channel_readv_full")
>     at /mnt/kvm/qemu/cleanup/qom/object.c:723
> #3  0x0000555555a3d4d7 in qio_channel_readv_full (ioc=0x5555568c5a00, 
> iov=0x7fff900008c0, niov=15, fds=0x0, nfds=0x0, errp=0x7fff9f9fe950)
>     at /mnt/kvm/qemu/cleanup/io/channel.c:56
> #4  0x0000555555a3ddc2 in qio_channel_readv (errp=0x7fff9f9fe950, 
> niov=<optimized out>, iov=<optimized out>, ioc=0x5555568c5a00)
>     at /mnt/kvm/qemu/cleanup/io/channel.c:197
> #5  0x0000555555a3ddc2 in qio_channel_readv_all_eof (ioc=0x5555568c5a00, 
> iov=<optimized out>, niov=<optimized out>, address@hidden)
>     at /mnt/kvm/qemu/cleanup/io/channel.c:106
> #6  0x0000555555a3de79 in qio_channel_readv_all (ioc=<optimized out>, 
> iov=<optimized out>, niov=<optimized out>, address@hidden)
>     at /mnt/kvm/qemu/cleanup/io/channel.c:142
> #7  0x0000555555794768 in multifd_recv_thread (opaque=0x5555570e5e00)
> ---Type <return> to continue, or q <return> to quit---up
>     at /mnt/kvm/qemu/cleanup/migration/ram.c:722
> #8  0x00007ffff2cc036d in start_thread (arg=0x7fff9f9ff700)
>     at pthread_create.c:456
> #9  0x00007ffff29f8bbf in clone ()
>     at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97
> (gdb) up
> 
> Removing this unref fixed things, so I think that the accounting is good
> (famous last words).

No, this is just papering over a bug elsewhere. The multifd_new_channel
metohd needs to be acquiring a reference for the multifd_recv_thread to
hold onto.


Regards,
Daniel
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