Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

From: Ingo Molnar
Date: Thu Nov 30 2006 - 03:32:13 EST

* Gautham R Shenoy <ego@xxxxxxxxxx> 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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