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


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