Re: [PATCH tip/core/rcu 0/3] rcu: prevent hangs by simplifying rcu_barrier/CPU-hotplug, fix lockdep complaint

Ingo Molnar
2009-10-07T06:16:01Z

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> This patchset fixes a couple of issues with TREE_PREEMPT_RCU:
>
> 1. The current implementation of rcu_barrier() waits for any
> prior CPU-hotplug operation to complete. This code path has
> resulted in some hangs, so first move this code from rcupdate.c
> to rcutree.c and then use a simpler algorithm that avoids the
> waiting, eliminating the possibility of such hangs.
>
> 2. The lockdep facility complains from time to time due to the
> fact that certain combinations of CPU-hotplug operations can
> cause the TREE_PREEMPT_RCU code to acquire the root rcu_node
> structure's lock while holding a leaf rcu_node structure's
> lock. This patch places the root rcu_node structure's lock
> into its own lockdep class to prevent this false positive.
>
> Ingo, please rewind tip/core/rcu to commit #135c8aea before applying this
> series, as these patches really need to go into 2.6.32. Reworked and
> retested rcutiny (for 2.6.33, as Linus requested) will follow in a
> few days. Or more patches for more bug fixes, as the case may be. ;-)
>
> b/kernel/rcupdate.c | 120 -----------------------------------
> b/kernel/rcutree.c | 120 +++++++++++++++++++++++++++++++++++
> b/kernel/rcutree.h | 11 ++-
> b/kernel/rcutree_plugin.h | 34 ++++++++++
> b/kernel/rcutree_trace.c | 5 -
> kernel/rcutree.c | 156 ++++++++++++++++++++++++----------------------
> 6 files changed, 248 insertions(+), 198 deletions(-)

Applied, thanks Paul!

I dropped these commits for now:

3ffea79: rcu: Make hot-unplugged CPU relinquish its own RCU callbacks
eddd962: rcu: Move rcu_barrier() to rcutree, make lightweight rcu_barrier() for rcutiny
a39e7d5: rcu-tiny: The Bloatwatch Edition, v6

We can apply rcu-tiny for .33 again, once the dust has settled.

Ingo