Re: [PATCH 3/4] RCU: preemptible RCU implementation

[Christoph Hellwig <hch@infradead.org>]

On Mon, Aug 28, 2006 at 09:42:22PM +0530, Dipankar Sarma wrote:
> From: Paul McKenney <paulmck@us.ibm.com>
> 
> This patch implements a new version of RCU which allows its read-side
> critical sections to be preempted. It uses a set of counter pairs
> to keep track of the read-side critical sections and flips them
> when all tasks exit read-side critical section. The details
> of this implementation can be found in this paper -
> 
> http://www.rdrop.com/users/paulmck/RCU/OLSrtRCU.2006.08.11a.pdf
> 
> This patch was developed as a part of the -rt kernel
> development and meant to provide better latencies when
> read-side critical sections of RCU don't disable preemption.
> As a consequence of keeping track of RCU readers, the readers
> have a slight overhead (optimizations in the paper).
> This implementation co-exists with the "classic" RCU
> implementations and can be switched to at compiler.

NACK.  While a readers can sleep rcu version definitly has it's
we should make it all or nothing.  Either we always gurantee that
a rcu reader can sleep or never without external patches.  Having
this a config option is the ultimate defeat for any kind of bug
reproducabilility.

Please make the patch undconditional and see if it doesn't cause
any significant slowdowns in production-like scenaries and then
we can switch over to the readers can sleep variant unconditionally
at some point.