Re: [PATCH 1/2] lockdep: Include recursive read-locks dependencies in the tree

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2009-11-18 at 02:06 +0100, Frederic Weisbecker wrote:
> Currently, recursive read locks are checked in two ways:
> 
> - walk through the locks held by the current task and check possible
>   deadlock.
> 
> - if the recursive read lock is not already present in the lock held
>   by the current task, check its dependencies against the tree.
> 
> But this recursive read lock will never be added to the tree of
> dependencies. It means that the following sequence:
> 
> A = rwlock (Ar: taken as read recursive, Aw: taken as write)
> B = normal lock
> 
>  Ar -> B
>  B  -> Aw
> 
> won't ever be detected as a lock inversion.
> This patch fixes it by inserting the recursive read locks into the
> tree of dependencies and enhancing the circular checks (check the
> class and the read attribute collision).

There were some very funny corner cases with IRQ state vs recursive
locks, I don't seen any of that mentioned here.

Bot ego and I poked at it at various times, but neither of us managed to
actually finish it due to getting distracted with other bits I guess.

http://programming.kicks-ass.net/kernel-patches/cpu-hotplug/

http://lkml.org/lkml/2009/5/11/203