Re: [Qemu-devel] [PATCH 03/20] char: add IOWatchPoll support

[Amit Shah <amit.shah@redhat.com>]

On (Fri) 29 Mar 2013 [09:03:10], Anthony Liguori wrote:
> Amit Shah <amit.shah@redhat.com> writes:
> 
> > On (Fri) 29 Mar 2013 [07:24:07], Anthony Liguori wrote:
> >> Amit Shah <amit.shah@redhat.com> writes:
> >> 
> >> > On (Tue) 05 Mar 2013 [23:21:18], Amit Shah wrote:
> >> >> From: Anthony Liguori <aliguori@us.ibm.com>
> >> >> 
> >> >> This is a special GSource that supports CharDriverState style
> >> >> poll callbacks.
> >> >> 
> >> >> For reviewability and bisectability, this code is #if 0'd out in this
> >> >> patch to avoid unused warnings since all of the functions are static.
> >> >> 
> >> >> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
> >> >> Signed-off-by: Amit Shah <amit.shah@redhat.com>
> >> >
> >> >
> >> >> +static int io_channel_send_all(GIOChannel *fd, const void *_buf, int len1)
> >> >> +{
> >> >> +    GIOStatus status;
> >> >> +    gsize bytes_written;
> >> >> +    int len;
> >> >> +    const uint8_t *buf = _buf;
> >> >> +
> >> >> +    len = len1;
> >> >> +    while (len > 0) {
> >> >> +        status = g_io_channel_write_chars(fd, (const gchar *)buf, len,
> >> >> +                                          &bytes_written, NULL);
> >> >> +        if (status != G_IO_STATUS_NORMAL) {
> >> >> +            if (status != G_IO_STATUS_AGAIN) {
> >> >> +                return -1;
> >> >> +            }
> >> >
> >> > It's not quite right to return -1 here; previous iterations of the
> >> > while loop could have successfully written data, and (len1 - len)
> >> > could be +ve.
> >> 
> >> Once -1 is returned, it's a terminal error.  It doesn't matter that we
> >> may have written some data.
> >
> > Why do you say that?
> 
> Because you're quoting the wrong patch :-)

Indeed.

>  This bit is rewritten by a
> later patch which is the source of your problem below.  In the patch you
> quote, we busy spin until all data is written.  However, with:
> 
>     commit 23673ca740e0eda66901ca801a5a901df378b063
>     Author: Anthony Liguori <aliguori@us.ibm.com>
>     Date:   Tue Mar 5 23:21:23 2013 +0530
>     
>         qemu-char: add watch support
> 
> We started to return EAGAIN even if we have a partially successful
> write.  I'm running a patch through testing right now that rewrites this
> function to have sane semantics (return bytes written on partial write).

Yes, that's where the problem is: EINTR and EAGAIN returns.

> I'll post as soon as testing completes.

Thanks!

		Amit