Messages in this thread | | | Date | Thu, 30 Nov 2006 09:29:34 +0100 | From | Ingo Molnar <> | Subject | Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency |
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* Gautham R Shenoy <ego@in.ibm.com> wrote:
> Ok, I see that we are already doing it :(. So we can end up in a > deadlock. > > Here's the culprit callpath:
in general lockdep is 100% correct when it comes to "individual locks". The overwhelming majority of lockdep false-positives is not due to lockdep not getting the dependencies right, but due to the "lock class" not being correctly identified. That's not an issue here i think.
what lockdep does is it observes actual locking dependencies as they happen individually in various contexts, and then 'completes' the dependency graph by combining all the possible scenarios how contexts might preempt each other. So if lockdep sees independent dependencies and concludes that they are circular, there's nothing that saves us from the deadlock.
The only way for those dependencies to /never/ trigger simultaneously on different CPUs would be via the use of a further 'outer' exclusion mechanism (i.e. a lock) - but all explicit kernel-API exclusion mechanisms are tracked by lockdep => Q.E.D. (Open-coded exclusion might escape the attention of lockdep, but those are extremely rare and are also easily found.)
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