Re: [Qemu-devel] Questions about nbd with QIOChannel

[Paolo Bonzini]

On 07/04/2016 13:04, Changlong Xie wrote:
> Hi all
> 
> Recently during test COLO, i found sometimes the client goes to hung on
> Primary side. First i thought it maybe a COLO revelant issue, but after
> ton of tests i doubt that this maybe a NBD issue (athough i'm not sure).
> So i'd like to share what i found:
> 
> Since commit 1c778ef7, we convert to using QIOChannel APIs for actual
> socket I/O.
> 
> nbd_reply_ready()
>   read_sync()
>     nbd_wr_syncv()
>     {
>      ...
>      while (nlocal_iov > 0) {
>          ...
>          if (do_read) {
>              len = qio_channel_readv(ioc, local_iov, nlocal_iov,
>                                      &local_err);
>          } else {
>              ...
>          }
>          if (len == QIO_CHANNEL_ERR_BLOCK) {
>              if (qemu_in_coroutine()) {
>                  qemu_coroutine_yield();
>              } else {
>                  qio_channel_wait(ioc,
>                                   do_read ? G_IO_IN : G_IO_OUT);
>              }

You are right; you've found a bug.

Paolo