Re: [Xen-devel] Basic blktap2 functionality issues.

["Dr. Greg Wettstein" <greg@xxxxxxxxxxxxxxxxx>]

On Apr 2,  4:51pm, Ian Jackson wrote:
} Subject: Re: [Xen-devel] Basic blktap2 functionality issues.

Hi Ian, et. al, hope this note finds your day going well.

> Ian Campbell writes ("Re: [Xen-devel] Basic blktap2 functionality issues."):
> > On Fri, 2012-03-30 at 09:17 +0100, Ian Campbell wrote:
> > > I think an approach worth trying would be to have
> > > tapdisk_control_detach_vbd respond to TAPDISK_MESSAGE_DETACH before
> > > doing the actual detach. i.e. it would respond with "Yes, I will do
> > > that" rather than "Yes, I have done that". My speculation is that this
> > > will allow libxl to continue and hopefully avoid the deadlock.
> > 
> > This seems to be the case as the following fixes things for me. Thanks
> > very much for your analysis which lead me to this solution...

> Greg, can you confirm whether this works for you ?

I've been grinding through the blktap2 issues and came up with some
additional findings I wanted to bounce off everyone.

The ability to reproduce this problem with the tap-ctl utility helped
confirm what the problem 'smelled like' from the beginning, ie. a hung
reference to the tapdev device instance which is ultimately preventing
the .release method from completing on the tapdisk2 side of things.

I started snooping around in xenstore and I believe I am seeing an
issue where xl does not release the the vbd instance in dom0.  This
causes a steady and persistent increase in the number of 'stale' vbd
instances held in dom0.  At a minimum this would suggest that xl is
not properly releasing resources properly which may help track the
whole issue down.

I replaced the tap: directive with a phy: device instance and saw the
orphan vbd instance as well so the problem doesn't seem related to a
blktap driver instance.

This is with the original patch applied which 'fixed' the problem with
the tapdisk2 process being left alive.  I'm going to back out that
patch to see if it reproduces with stock 4.1.2.

In the meantime let me know if anyone else is seeing this or if anyone
remembers a generic problem like this being fixed in xl in unstable.

> Ian.

Have a good afternoon.

}-- End of excerpt from Ian Jackson

As always,
Dr. G.W. Wettstein, Ph.D.   Enjellic Systems Development, LLC.
4206 N. 19th Ave.           Specializing in information infra-structure
Fargo, ND  58102            development.
PH: 701-281-1686
FAX: 701-281-3949           EMAIL: greg@xxxxxxxxxxxx
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