On Oct 31, 2018, at 2:00 PM, Peter Zijlstra [off-list ref] wrote:

quoted

On Wed, Oct 31, 2018 at 01:36:48PM -0700, Andy Lutomirski wrote:

quoted

quoted

On Oct 31, 2018, at 3:02 AM, Peter Zijlstra [off-list ref] wrote:

On Tue, Oct 30, 2018 at 09:41:13PM -0700, Andy Lutomirski wrote:
To clarify some of this thread, I think that the fact that rare_write
uses an mm_struct and alias mappings under the hood should be
completely invisible to users of the API.  No one should ever be
handed a writable pointer to rare_write memory (except perhaps during
bootup or when initializing a large complex data structure that will
be rare_write but isn't yet, e.g. the policy db).

Being able to use pointers would make it far easier to do atomics and
other things though.

This stuff is called *rare* write for a reason. Do we really want to
allow atomics beyond just store-release?  Taking a big lock and then
writing in the right order should cover everything, no?

Ah, so no. That naming is very misleading.

We modify page-tables a _lot_. The point is that only a few sanctioned
sites are allowed writing to it, not everybody.

I _think_ the use-case for atomics is updating the reference counts of
objects that are in this write-rare domain. But I'm not entirely clear
on that myself either. I just really want to avoid duplicating that
stuff.

Sounds nuts. Doing a rare-write is many hundreds of cycles at best. Using that for a reference count sounds wacky.

Can we see a *real* use case before we over complicate the API?