Re: [Xen-devel] [PATCH] cxenstored: correct calculation of data/space in the ring

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Mon, 2015-11-16 at 18:36 +0000, Wei Liu wrote:

Ian, were your concerns addressed by this threadlet?

> On Mon, Nov 16, 2015 at 06:09:54PM +0000, Ian Jackson wrote:
> > Andrew Cooper writes ("Re: [PATCH] cxenstored: correct calculation of
> > data/space in the ring"):
> > > On 16/11/15 18:01, Ian Jackson wrote:
> > > > Wei Liu writes ("[PATCH] cxenstored: correct calculation of
> > > > data/space in the ring"):
> > > > > The cxenstored implementation can't handle cross ring boundary
> > > > > read and
> > > > > write. It gets aways with buggy behaviour because upper layer
> > > > > won't
> > > > > sleep when short-write or short-read occurs.
> > > > I don't understand why you think this is a bug.
> > > 
> > > It is exactly the same bug as I fixed in c/s 8a2c11f8
> > > 
> > > The short reads/writes themselves aren't inherently a problem.ÂÂThey
> > > are
> > > genuine signals that the server should wait for the client to
> > > produce/consume more data.
> > > 
> > > However, the low level functions erroneously return a short
> > > read/write
> > > when hitting the ring boundary when there is actually more
> > > space/data. 
> > > This causes a protocol stall as the server incorrectly believes that
> > > the
> > > client has the next action to perform.
> > 
> > If I understand Wei correctly you are contradicting him.ÂÂThe `upper
> > layer' in question is inside the C xenstored so there is no protocol
> > stall.
> > 
> 
> There is no protocol stall for now. But the code that controls whether
> to sleep or not can change (however unlikely). And it would be hard to
> debug such bug as the effort for debugging stubdom / oxenstored already
> demonstrated.
> 
> IMO short-writing and short-reading when there is still space / data is
> a bug in its own right. We might as well just fix it before we get hit
> again.
> 
> Wei.
> 
> > (I haven't peered at the code...)
> > 
> > Ian.

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