On Tue, 4 Oct 2011, Paul E. McKenney wrote: > On Wed, Oct 05, 2011 at 12:05:47AM +0200, Thomas Gleixner wrote: > > > This means that might_sleep() will never complain about > > > blocking in an RCU read-side critical section. I guess that > > > this is necessary, though it would be better to have some > > > way to complain for general sleeping (e.g., waiting on > > > network receive) as opposed to blocking on a lock that is > > > subject to priority inheritance. > > > > Well, there's always a remaining problem. We need that stuff fully > > preemptible on rt. Any ideas ? > > Not yet. We would have to classify context switches into two groups: > > 1. Preemptions or blocking waiting for sleeping spinlocks. > > 2. Everything else. > > Given that classification, it would be straightforward: prohibit > group #2 context switches while in RCU-preempt read-side critical > sections. I know, easy for me to say! ;-) Well, you know the preemtible regions of RT, it basically boils down to #1 - except that it differs a bit from vanilla that locks and bh stuff does not prevent preemption. If RCU can deal with that, then #2 is a non issue :) > > > rcu-disable-the-rcu-bh-stuff-for-rt.patch > > > > > > This implements RCU-bh in terms of RCU-preempt, but disables > > > BH around the resulting RCU-preempt read-side critical section. > > > May be vulnerable to network-based denial-of-service attacks, > > > which could OOM a system with this patch. > > > > > > The implementation of rcu_read_lock_bh_held() is weak, but OK. In > > > an ideal world, it would also complain if not local_bh_disable(). > > > > Well, I dropped that after our IRC conversation, but we still need to > > have some extra debugging for RT to diagnose situations where we break > > those rcu_bh assumptions. That _bh rcu stuff should have never been > > there, we'd rather should drop the softirq processing back to > > ksoftirqd in such an overload case (in mainline) and voluntary > > schedule away from ksoftirqd until the situation is resolved. > > > > I consider rcu_bh a bandaid for the nasty implict semantics of BH > > processing and I'm looking really forward to Peters analysis of the > > modern cpu local BKL constructs at RTLWS. > > > > The patch stemmed from an earlier discussion about getting rid of > > those special rcu_bh semantics even in mainline for the sake of not > > making a special case for a completely over(ab)used construct which > > originates from the historically grown softirq behaviour. I think that > > keeping the special cased rcu_bh stuff around is just taking any > > incentive away from moving that whole softirq processing into a > > scheduler controllable environment (i.e. threaded interrupts). > > Between -rt and the guys pushing packets, I can tell that this is going > to be a fun one. ;-) We'll see. At some point they'll find out that a thread context will make their life easier simply because the locking maze can be distangled. That's why I'm ranting at the special "foster _bh" rcu abominations, which keep them thinking that going down that road is actually a good thing. > I will see if I can come up with a way to make that patch safe to > apply. Might not be all that hard. :) Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
