rwlocks, correctness over speed.

[Max Laier]

On Thursday 22 November 2007, Attilio Rao wrote:
> 2007/11/22, Max Laier <max at love2party.net>:
> > rwlocks are already used in places that do recursive reads.  The one
> > place I'm certain about is pfil(9) and we need a proper sollution for
> > this. Before rwlocks were used, I had a handrolled locking that
> > supported both read/write semantics and starvation avoidance - at the
> > cost of failing to allow futher read access when a writer asked for
> > access.  This however, was quite application specific and not the
> > most efficient implementation either.
>
> I'm not a pfil(9) expert, but for what I've heard, rmlocks should be
> really good for it, shouldn't them?
>
> The concept is that if we want to maintain fast paths for rwlock we
> cannot do too much tricks there. And you can really deadlock if you
> allow recursion on readers...

How about adding rwlock_try_rlock() which would do the following:
 1) Only variant to allow[1] read recursion and ...
 2) ... only if no outstanding write requests
 3) Let the caller deal with failure

This can be implemented statically, so no overhead in the fast path.  The 
caller is in the best position to decide if it is recursing or not - 
could keep that info on the stack - and can either fail[2] or do a normal 
rwlock_rlock() which would wait for the writer to enter and exit.

[2] In most situation where you use read locks you can fail or roll back 
carefully as you didn't change anything - obviously.  In pfil - for 
instance - we just dropped the packet if there was a writer waiting.

[1] "allow" in terms of WITNESS - if that can be done.

> > If we were to disallow read recursion, we should have some generic
> > lock type that does allow it.  rmlock(9)s seem to support full
> > priority propagation even for recursed readers.  Can they be MFCed so
> > that we have an alternative?  Are they considered ready for
> > production?  Should we switch pfil(9) to them?  It seems like a
> > perfect match.
>
> I've not looked over rmlocks so much, but as they track readers they
> can do priority propagation (if it is not implemented, it could be
> done -- not sure about the cost of the operation though).
>
> Attilio



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