Re: [PATCH rcu/urgent 0/6] Fixes for RCU/scheduler/irq-threads trainwreck

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jul 20, 2011 at 08:52:58PM +0200, Peter Zijlstra wrote:
> On Wed, 2011-07-20 at 20:44 +0200, Ingo Molnar wrote:
> > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > > RCU boost is new and counts as strange for another few days at least.
> > 
> > I'd rather prefer a oneliner patch that turns it off in the Kconfig 
> > for now.
> > 
> > We are awfully late in the -rc cycle and this diffstat:
> > 
> >  b/include/linux/sched.h   |    3 ++
> >  b/kernel/rcutree_plugin.h |    6 +++--
> >  b/kernel/sched.c          |   44 ++++++++++++++++++++++++++++++++++-----
> >  b/kernel/signal.c         |   19 +++++++++++------
> >  b/kernel/softirq.c        |   12 +++++++++-
> >  kernel/rcutree_plugin.h   |   51 +++++++++++++++++++++++++++++++++-------------
> >  6 files changed, 105 insertions(+), 30 deletions(-)
> > 
> > looks very scary to me.
> 
> A lot of that is also relevant to !BOOST.

Unfortunately, agreed.  :-(

The expedited grace periods also cause rcu_read_unlock() to call
into the scheduler.  This can interact badly with the recently
added RCU read-side critical sections in the scheduler that have
either the runqueue or the priority-inheritance locks held, especially
when interrupts occur towards the end of __rcu_read_unlock().  But
there are some failure cases that don't involve interrupts (though
I believe all of them involve RCU_BOOST).

That said, turning off RCU_BOOST would allow dropping this patches:

7765be2fec0f4 Fix RCU_BOOST race handling current->rcu_read_unlock_special



							Thanx, Paul