Re: [ANNOUNCE] Linux 2.6 Real Time Kernel

[Dipankar Sarma]

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

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. My primary concern is
DoS/OOM situation due to preempted tasks holding up RCU.

> 
> could you send those RCU patches (no matter how incomplete/broken)? It's
> the main issue that causes the dcache_lock to be raw still. (and a
> number of dependent locks: fs-writeback.c, inode.c, etc.) We can make
> those RCU changes not impact the normal !PREEMPT_REALTIME locking so it
> might have a chance for upstream merging as well.

I would be interested in this too. 

Thanks
Dipankar
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