Re: Real-time rw-locks (Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.32-15)

[Ingo Molnar <mingo@elte.hu>]

* Esben Nielsen <simlo@phys.au.dk> wrote:

> I noticed that you changed rw-locks to behave quite diferently under
> real-time preemption: They basicly works like normal locks now. I.e.
> there can only be one reader task within each region. This can can
> however lock the region recursively. [...]

correct.

> [...] I wanted to start looking at fixing that because it ought to
> hurt scalability quite a bit - and even on UP create a few unneeded
> task-switchs. [...]

no, it's not a big scalability problem. rwlocks are really a mistake -
if you want scalability and spinlocks/semaphores are not enough then one
should either use per-CPU locks or lockless structures. rwlocks/rwsems
will very unlikely help much.

> However, the more I think about it the bigger the problem:

yes, that complexity to get it perform in a deterministic manner is why
i introduced this (major!) simplification of locking. It turns out that
most of the time the actual use of rwlocks matches this simplified
'owner-recursive exclusive lock' semantics, so we are lucky.

look at what kind of worst-case scenarios there may already be with
multiple spinlocks (blocker.c). With rwlocks that just gets insane.

	Ingo