Re: [ANNOUNCE] Linux 2.6 Real Time Kernel

[Paul E. McKenney]

On Fri, Oct 15, 2004 at 05:45:42PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> 
> > One caution (which you are no doubt already aware of) -- if an RCU
> > algorithm that reads (rcu_read_lock()/rcu_read_unlock()) in process
> > context and updates in softirq/bh/irq context, you can see deadlocks.
> 
> yeah - but in the PREEMPT_REALTIME kernel there are simply no irq or
> softirq contexts in process contexts - everything is a task. So
> everything can (and does) block.

OK, am probably confused, but I thought that the whole point of your
PREEMPT_REALTIME implementation of rcu_read_lock_rt() was to enable
preemption in the RCU read-side critical section.  If this is indeed
the case, then it looks to me like code that would run in softirq/bh/irq
context in a kernel compiled non-PREEMPT_REALTIME could now run during
the time that a code path running under rcu_read_lock_rt() was preempted.

If so, then the kernel can end up freeing a data item that the preempted
RCU read-side critical section is still referencing.

OK, so what am I missing here?

						Thanx, Paul
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