Re: Panic when cpu hot-remove

From: Alex Williamson
Date: Wed Jun 17 2015 - 10:36:48 EST

On Wed, 2015-06-17 at 13:52 +0200, Joerg Roedeljoro wrote:
> On Wed, Jun 17, 2015 at 10:42:49AM +0000, èåå wrote:
> > Hi maintainer,
> >
> > We found a problem that a panic happen when cpu was hot-removed. We also trace the problem according to the calltrace information.
> > An endless loop happen because value head is not equal to value tail forever in the function qi_check_fault( ).
> > The location code is as follows:
> >
> >
> > do {
> > if (qi->desc_status[head] == QI_IN_USE)
> > qi->desc_status[head] = QI_ABORT;
> > head = (head - 2 + QI_LENGTH) % QI_LENGTH;
> > } while (head != tail);
> Hmm, this code interates only over every second QI descriptor, and tail
> probably points to a descriptor that is not iterated over.
> Jiang, can you please have a look?

I think that part is normal, the way we use the queue is to always
submit a work operation followed by a wait operation so that we can
determine the work operation is complete. That's done via
qi_submit_sync(). We have had spurious reports of the queue getting
impossibly out of sync though. I saw one that was somehow linked to the
I/O AT DMA engine. Roland Dreier saw something similar[1]. I'm not
sure if they're related to this, but maybe worth comparing. Thanks,



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