Re: [Xen-devel] Zero-sized reads from XenBus block

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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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