* David Gibson (da...@gibson.dropbear.id.au) wrote:
> On Wed, Feb 25, 2015 at 04:51:31PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
> >
> > The return path uses a non-blocking fd so as not to block waiting
> > for the (possibly broken) destination to finish returning a message,
> > however we still want outbound data to behave in the same way and block.
>
> It's not clear to me from this description exactly where the situation
> is that you need to write to the non-blocking socket. Is it on the
> source or the destination? If the source, why are you writing to the
> return path? If the destination, why are you marking the outgoing
> return path as non-blocking?
My understanding is that the semantics of set_nonblock() are to
set non-blocking on all operations on the transport associated with
the fd - and that it's true even if you dup() the fd; and so if you
set non-blocking in one direction you get it in the other direction as well.
The (existing) destination side sets non-block (see process_incoming_migration
in migration.c), and so it gets non-blocking on the incoming data stream, but
that has the side effect that it's also going to be non-blocking on
the destinations writes to the reverse-path; thus we need to be able
to safely do writes from the destination reverse-path.
The text is out of date, back in v2 the source used to use non-blocking
for the return-path, but we managed to kill that off by using a thread
for the return path in the source.
How about changing the text to:
--------
The destination sets the fd to non-blocking on incoming migrations;
this also affects the return path from the destination, and thus we
need to make sure we can safely write to the return path.
Dave
>
> > Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> > ---
> > migration/qemu-file-unix.c | 41 ++++++++++++++++++++++++++++++++++++-----
> > 1 file changed, 36 insertions(+), 5 deletions(-)
> >
> > diff --git a/migration/qemu-file-unix.c b/migration/qemu-file-unix.c
> > index 50291cf..218dbd0 100644
> > --- a/migration/qemu-file-unix.c
> > +++ b/migration/qemu-file-unix.c
> > @@ -39,12 +39,43 @@ static ssize_t socket_writev_buffer(void *opaque,
> > struct iovec *iov, int iovcnt,
> > QEMUFileSocket *s = opaque;
> > ssize_t len;
> > ssize_t size = iov_size(iov, iovcnt);
> > + ssize_t offset = 0;
> > + int err;
> >
> > - len = iov_send(s->fd, iov, iovcnt, 0, size);
> > - if (len < size) {
> > - len = -socket_error();
> > - }
> > - return len;
> > + while (size > 0) {
> > + len = iov_send(s->fd, iov, iovcnt, offset, size);
> > +
> > + if (len > 0) {
> > + size -= len;
> > + offset += len;
> > + }
> > +
> > + if (size > 0) {
> > + err = socket_error();
> > +
> > + if (err != EAGAIN) {
> > + error_report("socket_writev_buffer: Got err=%d for
> > (%zd/%zd)",
> > + err, size, len);
> > + /*
> > + * If I've already sent some but only just got the error, I
> > + * could return the amount validly sent so far and wait
> > for the
> > + * next call to report the error, but I'd rather flag the
> > error
> > + * immediately.
> > + */
> > + return -err;
> > + }
> > +
> > + /* Emulate blocking */
> > + GPollFD pfd;
> > +
> > + pfd.fd = s->fd;
> > + pfd.events = G_IO_OUT | G_IO_ERR;
> > + pfd.revents = 0;
> > + g_poll(&pfd, 1 /* 1 fd */, -1 /* no timeout */);
> > + }
> > + }
> > +
> > + return offset;
> > }
> >
> > static int socket_get_fd(void *opaque)
>
> --
> David Gibson | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_
> _other_
> | _way_ _around_!
> http://www.ozlabs.org/~dgibson
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
