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Re: [Xen-devel] dom0 - BUG: unable to handle kernel paging req

To: "Christopher S. Aker" <caker@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] dom0 - BUG: unable to handle kernel paging request
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 31 Jan 2011 14:22:47 -0800
Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 01/31/2011 01:07 PM, Christopher S. Aker wrote:
>> Xen: 3.4.4-rc1-pre 64bit (xenbits @ 19986)
> > Dom0: PAE (xen/stable-2.6.32.x)
>> We've been running our xen-thrash testsuite on a bunch of hosts
>> against a very recent build, and we've just hit this on one box:
>> BUG: unable to handle kernel paging request at 15555d60
> Two additional boxes out of my last test round have also hit this.
> About one a week.
> Ian / Jeremy:  Where do I go from here?

There seems to be a moderately difficult-to-hit (but still pretty large)
race in pagetable teardown.  It *should* be protected by the pgd lock,
so we need to work out where a teardown (or access) is happening without
that lock.  I think that's going to be a matter of close code-review
rather than any more testing.

The interesting thing is that this problem seems to have come to the
fore since the the patch that was explicitly intended to avoid it was
put in :/...  Before that, the race was theoretical, but AFAIK had never
been observed in a pvops kernel (though it was seen in the Citrix
product in non-pvops kernels, which is why we fixed it).

I'll try to stare at it in the next couple of days.


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