Re: [Libguestfs] [PATCH libnbd v2 3/4] api: Implement concurrent writer.

There are several races / deadlocks which I've thought about. Let's see if I can remember them all ... (1) This I experienced: nbd_aio_get_fd deadlocks if there are concurrent synchronous APIs going on. A typical case is where you set up the concurrent writer thread before connecting, and then call a synchronous connect function such as connect_tcp. The synchronous function grabs h->lock, then writes something, which eventually invokes the writer thread which calls nbd_aio_get_fd and deadlocks on h->lock. -> Probably the writer thread should be forbidden from using nbd_handle. (2) The writer thread calls nbd_aio_get_fd, but the fd returned might might be closed before we use it, resulting in either EBADF or worse using another fd that happens to be opened around the same time. -> I think the solution to this would be to allow the writer callback to signal that the socket is about to be closed (eg. add an extra flag parameter to the callback), which would kill the writer thread. (3) nbd_concurrent_writer_error could lose errors. This might happen if the socket is closed normally without writing anything, which would never check h->writer_error. (4) nbd_concurrent_writer_error possibly deadlocks too since it needs to grab h->lock. Basically the same as (1). Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v