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    SubjectRe: [ANNOUNCE] Linux 2.6 Real Time Kernel
    On Thu, Oct 14, 2004 at 09:18:10AM +0200, Ingo Molnar wrote:
    >
    > * Dipankar Sarma <dipankar@in.ibm.com> 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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