Re: [Qemu-devel] [PATCH] migrate_fd_close: delete associated io-handler before closing the fd

[Mark McLoughlin <markmc@redhat.com>]

On Mon, 2009-06-08 at 14:28 +0300, Uri Lublin wrote:
> It may happen that the io-handler is still registered. That causes
> select() to return with EBADF, not calling handlers for other fds.
> 
> The io-handler would be registered when (on the source) the whole state
> was written but not yet flushed. For example when using QEMUFileBuffered,
> (tcp-migration) there may be data left in a buffer waiting to be transferred.
> In such a case buffered_close() calls buffered_flush() which calls
> migrate_fd_put_buffer, which may, upon EAGAIN, register migrate_fd_put_notify
> as a handler.
> 
> Signed-off-by: Uri Lublin <uril@redhat.com>
> ---
>  migration.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/migration.c b/migration.c
> index 401383c..57f2a52 100644
> --- a/migration.c
> +++ b/migration.c
> @@ -301,5 +301,7 @@ void migrate_fd_wait_for_unfreeze(void *opaque)
>  int migrate_fd_close(void *opaque)
>  {
>      FdMigrationState *s = opaque;
> +
> +    qemu_set_fd_handler2(s->fd, NULL, NULL, NULL, NULL);

Looks good, but perhaps a comment explaining how the I/O handler could
possibly be registered here would be useful - at first glance, it seemed
to me that the I/O handler should always be de-registered in
migrate_fd_cleanup() before getting here.

The key to understanding the problem is that qemu_fclose() on a buffered
file can cause I/O to be flushed.

Reviewed-by: Mark McLoughlin <markmc@redhat.com>

Cheers,
Mark.