Re: [Qemu-devel] [patch] Fix block I/O hang.

[Gerd Hoffmann <kraxel@redhat.com>]

Anthony Liguori wrote:

> Yes, this all is.  I think the intention was to make it non-blocking so
> that the full queue could be drained to avoid superfluous calls to
> posix_aio_read() since it handles all outstanding ops at once.

That is what I concluded after a brief look, without realizing that the
loop doesn't actually do that (yet?), thus went for the "oops, this fd
must be in non-blocking mode" fix.

>  However,
> 
>> Third, when called from a select callback it shouldn't block indeed.  It
>> does though for me now and then when booting xen guests (with a big
>> stack of xenner patches).  Doesn't reproduce reliable though.  Sprinkled
>> in a printk (with rfd being non-blocking) and got a EAGAIN once, so it
>> got called with rfd not having data.
> 
> I think it would be good to root cause this.  AFAICT, it shouldn't block.

Yes, something is fishy here.  Will try to pin it.
Ideas where to look are welcome in case you have any ;)

cheers,
  Gerd