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


Btw, for user space, if you want to do this all right for something like
BF. I think the only _correct_ thing to do (in the sense that the end
result will actually be debuggable) is to essentially give full SMP
coherency in user space.

It's doable, but rather complicated, and I'm not 100% sure it really ends
up making sense. The way to do it is to just simply say:

- never map the same page writably on two different cores, and always
flush the cache (on the receiving side) when you switch a page from one
core to another.

Now, the kernel can't really do that reasonably, but user space possibly could.

Now, I realize that blackfin doesn't actually even have a MMU or a TLB, so
by "mapping the same page" in that case we end up really meaning "having a
shared mapping or thread". I think that _should_ be doable. The most
trivial approach might be to simply limit all processes with shared
mappings or CLONE_VM to core 0, and letting core 1 run everything else
(but you could do it differently: mapping something with MAP_SHARED would
force you to core 0, but threads would just force the thread group to
stay on _one_ core, rather than necessarily a fixed one).

Yeah, because of the lack of real memory protection, the kernel can't
_know_ that processes don't behave badly and access things that they
didn't explicitly map, but I'm hoping that that is rare.

And yes, if you really want to use threads as a way to do something
across cores, you'd be screwed - the kenrel would only schedule the
threads on one CPU. But considering the undefined nature of threading on
such a cpu, wouldn't that still be preferable? Wouldn't it be nice to have
the knowledge that user space _looks_ cache-coherent by virtue of the
kernel just limiting cores appropriately?

And then user space would simply not need to worry as much. Code written
for another architecture will "just work" on BF SMP too. With the normal
uclinux limitations, of course.

Linus


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