On Wed, Mar 27, 2013 at 07:37:15AM -0700, Paul E. McKenney wrote:
> On Wed, Mar 27, 2013 at 03:17:32PM +0100, Peter Zijlstra wrote:
> > On Wed, 2013-03-27 at 14:15 +0100, Joerg Roedel wrote:
> > > What makes me wonder here is that the code is preemptible in an
> > > rcu_read_locked section. As far as I know preemption needs to be
> > > disabled while holding the rcu_read_lock().
> >
> > Nah, a long long time ago some -rt people complained to paulmck that
> > keeping preemption disabled over all this RCU stuff was killing
> > latencies. Paul liked the challenge and came up with some mind twisting
> > stuff to make it work.
>
> What can I say? I was young and foolish. And I still am pretty
> foolish. ;-)
>
> But yes, you are not required to disable preemption across
> rcu_read_lock(), and rcu_read_lock() is not guaranteed to disable
> preemption. So if you need preemption to be disabled, do it explicitly
> with preempt_disable(), local_irq_save(), rcu_read_lock_sched(),
> or whatever, because rcu_read_lock() isn't always going to disable
> preemption.
>
> > If you're into that kind of pain, look at CONFIG_*_PREEMPT_RCU :-)
>
> Or just set CONFIG_PREEMPT=y, which will set CONFIG_TREE_PREEMPT_RCU=y
> on CONFIG_SMP=y builds and will set CONFIG_TINY_PREEMPT_RCU=y otherwise.
> (But please note that CONFIG_TINY_PREEMPT_RCU is going away, after which
> CONFIG_PREEMPT=y will always set CONFIG_TREE_PREEMPT_RCU=y.)
>
> Thanx, Paul
>
> > But yeah, you need to have that stuff enabled before you can hit this
> > particular snag.
Interesting read, thanks guys. I think I should have a look into the
tree-preempt implementation and try to understand it :)
Joerg
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