Re: [Xen-devel] [PATCH] x86: fix ordering of operations in destroy_irq()

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> George Dunlap <george.dunlap@xxxxxxxxxxxxx> 05/30/13 6:23 PM >>>
>On 05/29/2013 07:58 AM, Jan Beulich wrote:
>> The fix for XSA-36, switching the default of vector map management to
>> be per-device, exposed more readily a problem with the cleanup of these
>> vector maps: dynamic_irq_cleanup() clearing desc->arch.used_vectors
>> keeps the subsequently invoked clear_irq_vector() from clearing the
>> bits for both the in-use and a possibly still outstanding old vector.
>>
>> Fix this by folding dynamic_irq_cleanup() into destroy_irq(), which was
>> its only caller, deferring the clearing of the vector map pointer until
>> after clear_irq_vector().
>>
>> Once at it, also defer resetting of desc->handler until after the loop
>> around smp_mb() checking for IRQ_INPROGRESS to be clear, fixing a
>> (mostly theoretical) issue with the intercation with do_IRQ(): If we
>> don't defer the pointer reset, do_IRQ() could, for non-guest IRQs, call
>> ->ack() and ->end() with different ->handler pointers, potentially
>> leading to an IRQ remaining un-acked. The issue is mostly theoretical
>> because non-guest IRQs are subject to destroy_irq() only on (boot time)
>> error paths.
>>
>> As to the changed locking: Invoking clear_irq_vector() with desc->lock
>> held is okay because vector_lock already nests inside desc->lock (proven
>> by set_desc_affinity(), which takes vector_lock and gets called from
>> various desc->handler->ack implementations, getting invoked with
>> desc->lock held).
>>
>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
>How big of an impact is this bug?  How many people are actually affected 
>by it?

Andrew will likely be able to give you more precise info on this, but this
fixes a problem observed in practice. Any AMD system with IOMMU would
be affected.

>It's a bit hard for me to tell from the description, but it looks like 
>it's code motion, then some "theoretical" issues.

No, the description is pretty precise here: It fixes an actual issue and,
along the way, also a theoretical one.

>Is the improvement this patch represents worth the potential risk of 
>bugs at this point?

I think so - otherwise it would need to be backported right away after the
release.

Jan


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