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

[Gerd Hoffmann <kraxel@redhat.com>]

Anthony Liguori wrote:
> Gerd Hoffmann wrote:
>>   Hi,
>>
>> $subject says all.
>>
>> please apply,
>>   Gerd
> 
> Under what circumstances?  posix_aio_read() is only invoked from a
> select callback.  This means there should be data available to be read.

Hmm, looked at the loop again a bit closer.

First, the check for errno == EAGAIN is superfluous when the filehandle
isn't in non-blocking mode.  It will not happen.

Second, the check for errno == EINTR is superfluous too because the loop
will retry the read() syscall anyway for *any* error.

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.

cheers,
  Gerd