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

Paul E. McKenney
2009-10-07T13:53:17Z

On Wed, Oct 07, 2009 at 08:14:48AM +0200, Ingo Molnar wrote:
>
> * 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.

Sounds good! I am placing rcu-tiny and synchronize_srcu_expedited()
on a separate branch in my local git tree for 2.6.33.

Thanx, Paul