Re: Zero-sized reads from XenBus block

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 03/02/2016 11:35 AM, Roger Pau Monné wrote:
> El 2/3/16 a les 17:13, Wei Liu ha escrit:
>> CC Linux kernel and FreeBSD maintainers.
>>
>> On Wed, Mar 02, 2016 at 12:29:26AM +0300, Sergei Lebedev wrote:
>>> Hi list,
>>>
>>> I’m not sure if this is the expected behaviour, but it seems zero-sized reads from /dev/xen/xenbus block. Here’s sample code in Python
>>>
>>>      import os
>>>      
>>>      fd = os.open("/dev/xen/xenbus", os.O_RDWR)
>>>      os.read(fd, 0)  # Blocks.
>>>
>>> The issue is not language-specific, similar code in C blocks as well.
> I've tested your code on FreeBSD (after replacing /dev/xen/xenbus with
> /dev/xen/xenstore), and it doesn't block there. AFAICT this is because
> 0-size reads never get to the device "read" routine on FreeBSD, or else
> it would block.

This is how xenbus driver is designed --- it always blocks until 
something is written there.

It should indeed return zero right away but I wonder whether someone 
might count on current implementation (in the toolstack or elsewhere). 
Based on FreeBSD behavior I'd think this shouldn't be the case.


-boris

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