Re: [ANNOUNCE] Linux 2.6 Real Time Kernel

[Paul E. McKenney]

On Fri, Oct 15, 2004 at 09:40:39AM -0700, Paul E. McKenney wrote:
> 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?

Never mind!!!  You insert the mutex.  Sorry for the noise!

							Thanx, Paul
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