Re: [Xen-devel] BUG: using smp_processor_id() in preemptible [00000000] code: blkback.1.xvdb/9138 caller is decrease_reservation

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 11 Sep 2013, David Vrabel wrote:
> On 10/09/13 19:13, Sander Eikelenboom wrote:
> > Hi Wei,
> > 
> > Just back from holiday i tried running a new xen-unstable and linux
> > kernel (current Linus his tree + Konrad's last pull request merged on top).
> > I saw a thread and patch about a bug_on in increase_reservation ...
> > i'm seeing the splat below in dom0 when guests get started.
> 
> Yes, the use of __per_cpu() in decrease_reservation is not safe.
> 
> Stefano, what testing did you give "xen/balloon: set a mapping for
> ballooned out pages" (cd9151e2).  The number of critical problems it's
> had suggests not a lot?
> 
> I'm also becoming less happy with the inconsistency between the p2m
> updates between the different (non-)auto_translated_physmap guest types.
> 
> I think it (and all the attempts to fix it) should be reverted at this
> stage and we should try again for 3.13 which some more through testing
> and a more careful look at what updates to the p2m are needed.

Issues like this one are due to different kernel configurations / usage
patters. To reproduce this issue one needs a preemptible kernel and
blkback. I use a non-preemptible kernel and QEMU as block backend.

Granted, in this case I should have tested blkback and both preemptible
and non-preemptible kernel configurations.  But in general it is nearly
impossible to test all the possible configurations beforehand, it is a
classic case of combinatorial explosion.

These are exactly the kind of things that an exposure to a wider range of
users/developers help identify and fix.

So I think that we did the right thing here, by sending a pull request
early in the release cycle, so that now we have many other RCs to fix
all the issues that come up.

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