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RE: [Xen-devel] [PATCH] VMX: avoid taking locks with irqs disabled

>From: Keir Fraser
>Sent: Monday, October 20, 2008 6:09 PM
>Oh yes. That's a class of race that I hadn't realised would be
>introduced by
>the new rendezvous code. Spinlocks which are ever taken with
>IRQs disabled
>must *always* be taken with IRQs disabled. Spinlocks which are
>ever taken
>with IRQs enabled must *always* be taken with IRQs enabled.

We encountered a similar bug today, when Jimmy is developing
FSB HPET broadcast feature. The reason is same that above
guideline is not obeyed.

>It's quite an extra constraint on spinlock usage actually. :-(
>Actually this new partitioning of locks into two equivalence classes is
>begging for some run-time checking in debug builds. I'll sort
>out a patch
>for that.

Did you flush out the patch into xen bits? I guess there may
be other instances breaking that guideline. Just curious whether
Linux side already recognized similar constraints. It's possibly
not. Then another angle is, instead of pushing constraint on
spinlock usage, could we instead change timer rendezvous
style? Use softirq instead of call function should release the
constraints like stop_machine. :-)


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