Re: [PATCH 1/3] block/nbd: only detach existing iochannel from aio_context

[Roman Kagan]

On Fri, Jan 29, 2021 at 08:37:13AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 28.01.2021 23:14, Roman Kagan wrote:
> > When the reconnect in NBD client is in progress, the iochannel used for
> > NBD connection doesn't exist.  Therefore an attempt to detach it from
> > the aio_context of the parent BlockDriverState results in a NULL pointer
> > dereference.
> > 
> > The problem is triggerable, in particular, when an outgoing migration is
> > about to finish, and stopping the dataplane tries to move the
> > BlockDriverState from the iothread aio_context to the main loop.  If the
> > NBD connection is lost before this point, and the NBD client has entered
> > the reconnect procedure, QEMU crashes:
> > 
> >      at /build/qemu-gYtjVn/qemu-5.0.1/io/channel.c:452
> >      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6151
> >      new_context=new_context@entry=0x562a260c9580,
> >      ignore=ignore@entry=0x7feeadc9b780)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6230
> >      (bs=bs@entry=0x562a268d6a00, ctx=0x562a260c9580,
> >      ignore_child=<optimized out>, errp=<optimized out>)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/block.c:6332
> >      new_context=0x562a260c9580,
> >      update_root_node=update_root_node@entry=true, errp=errp@entry=0x0)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/block/block-backend.c:1989
> >      new_context=<optimized out>, errp=errp@entry=0x0)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/block/block-backend.c:2010
> >      out>)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/hw/block/dataplane/virtio-blk.c:292
> >      at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio-bus.c:245
> >      running=0, state=<optimized out>)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/hw/virtio/virtio.c:3220
> >      state=state@entry=RUN_STATE_FINISH_MIGRATE)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/softmmu/vl.c:1275
> >      send_stop=<optimized out>)
> >      at /build/qemu-gYtjVn/qemu-5.0.1/cpus.c:1032
> >      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:2914
> >      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3275
> >      at /build/qemu-gYtjVn/qemu-5.0.1/migration/migration.c:3439
> >      at /build/qemu-gYtjVn/qemu-5.0.1/util/qemu-thread-posix.c:519
> >      at pthread_create.c:333
> >      oldval=0x562a2452b138, oldlenp=0x0, newval=0x562a2452c5e0
> >      <__func__.28102>, newlen=0)
> >      at ../sysdeps/unix/sysv/linux/sysctl.c:30
> 
> function names are somehow lost :(

Oops.  Backtraces in gdb have frame numbers prefixed with a hash which
got interpreted as comments by git commit; I should have offset the
backtrace when pasting.

Will respin with fixed log messages, thanks.

Roman.