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

[Anthony Liguori <anthony@codemonkey.ws>]

Gerd Hoffmann wrote:
> 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.
>   

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.  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.

Regards,

Anthony Liguori


> cheers,
>   Gerd
>
>
>