Re: [ANNOUNCE] Linux 2.6 Real Time Kernel

[Paul E. McKenney]

On Thu, Oct 14, 2004 at 09:18:10AM +0200, Ingo Molnar wrote:
> 
> * Dipankar Sarma <dipankar@xxxxxxxxxx> wrote:
> 
> > On Tue, Oct 12, 2004 at 07:50:29AM +0200, Ingo Molnar wrote:
> > > 
> > > regarding RCU serialization - i think that is the way to go - i dont
> > > think there is any sensible way to extend RCU to a fully preempted
> > > model, RCU is all about per-CPU-ness and per-CPU-ness is quite limited 
> > > in a fully preemptible model.
> > 
> > It seems that way to me too. Long ago I implemented preemptible RCU,
> > but did not follow it through because I believed it was not a good
> > idea. The original patch is here :
> > 
> > http://www.uwsg.iu.edu/hypermail/linux/kernel/0205.1/0026.html
> 
> interesting!
> 
> > This allows read-side critical sections of RCU to be preempted. It
> > will take a bit of work to re-use it in RCU as of now, but I don't
> > think it makes sense to do so. [...]
> 
> note that meanwhile i have implemented another variant:
> 
>   http://marc.theaimsgroup.com/?l=linux-kernel&m=109771365907797&w=2
> 
> i dont think this will be the final interface (the _rt postfix is
> stupid, it should probably be _spin?), but i think this is roughly the
> structure of how to attack it - a minimal extension to the RCU APIs to
> allow for serialization. What do you think about this particular
> approach?

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.

						Thanx, Paul

> > [...] My primary concern is DoS/OOM situation due to preempted tasks
> > holding up RCU.
> 
> in the serialization solution in -U0 it would be possible to immediately
> free the RCU entries and hence have no DoS/OOM situation - although the
> -U0 patch does not do this yet.
> 
> 	Ingo
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