Re: [Uclinux-dist-devel] [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)

[Robin Getz <rgetz@blackfin.uclinux.org>]

On Thu 12 Feb 2009 15:13, Linus Torvalds pondered:
> On Thu, 12 Feb 2009, Paul E. McKenney wrote:
> > And, now that you mention it, I have heard rumors that other CPU 
> > families can violate cache coherence in some circumstances.
> 
> I personally suspect that the BF pseudo-SMP code is just broken, and
> that it likely has tons of subtle bugs and races - because we _do_ depend
> on cache coherency at least for accessing objects next to each other.

I felt similarly, however after using it, testing it, beating the crap out of 
it for while, and only finding a few niggly bugs which were corrected - it 
appears to be as stable as any other Linux kernel I have used on embedded 
hardware (which means rock solid).

There are a few people shipping it in their products today - so at least the 
stablily was also good enough for them.

If you have any other test suggestions which might expose the corner cases 
that normal use / LTP /  would not - I'm happy to try it out. The problem 
is - as Mike stated - since we do limit applications to one core at a time - 
multi-threading userspace isn't really interesting a problem.

As for the claim of "broken" hardware - I don't think that is true -- the 
hardware works as designed, as advertised. It was just never architected to 
run a SMP operating system. While you could claim that we are trying to force 
fit (SMP) something where it doesn't belong (non-cache coherence system), it 
is us that are "broken" - not the hardware :)

-Robin