Re: [PATCH RFC 0/4] memory: Fix (/ Discuss) a few rcu issues

[David Hildenbrand]

On 02.03.23 22:50, Peter Xu wrote:
> On Thu, Mar 02, 2023 at 04:11:56PM +0100, David Hildenbrand wrote:
> > I guess the main concern here would be overhead from gabbing/releasing the
> > BQL very often, and blocking the BQL while we're eventually in the kernel,
> > clearing bitmaps, correct?
>
> More or less yes.  I think it's pretty clear we move on with RCU unless
> extremely necessary (which I don't think..), then it's about how to fix the
> bug so rcu safety guaranteed.

What about an additional simple lock?

Like:

* register/unregister requires that new notifier lock + BQL
* traversing notifiers requires either that new lock or the BQL

We simply take the new lock in that problematic function. That would 
work as long as we don't require traversal of the notifiers concurrently 
-- and as long as we have a lot of bouncing back and forth (I don't 
think we have, even in the migration context, or am I wrong?).

That way we also make sure that each notifier is only called once. I'm 
not 100% sure if all notifiers would expect to be called concurrently.

--
Thanks,

David / dhildenb