Re: Real-Time Preemption and RCU

[Bill Huey (hui) <bhuey@lnxw.com>]

On Tue, Mar 22, 2005 at 02:04:46AM -0800, Bill Huey wrote:
> RCU isn't write deterministic like typical RT apps are[, so] we can... (below :-))
... 
> Just came up with an idea after I thought about how much of a bitch it
> would be to get a fast RCU multipule reader semantic (our current shared-
> exclusive lock inserts owners into a sorted priority list per-thread which
> makes it very expensive for a simple RCU case[,] since they are typically very
> small batches of items being altered). Basically the RCU algorithm has *no*
> notion of writer priority and to propagate a PI operation down all reader[s]
> is meaningless, so why not revert back to the original rwlock-semaphore to
> get the multipule reader semantics ?

The original lock, for those that don't know, doesn't strictly track read owners
so reentrancy is cheap.

> A notion of priority across a quiescience operation is crazy anyways[-,-] so
> it would be safe just to use to the old rwlock-semaphore "in place" without
> any changes or priorty handling add[i]tions. The RCU algorithm is only concerned
> with what is basically a coarse data guard and it isn't time or priority
> critical.

A little jitter in a quiescence operation isn't going to hurt things right ?. 

> What do you folks think ? That would make Paul's stuff respect multipule
> readers which reduces contention and gets around the problem of possibly
> overloading the current rt lock implementation that we've been bitching
> about. The current RCU development track seem wrong in the first place and
> this seem like it could be a better and more complete solution to the problem.

Got to get rid of those typos :)

bill