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

[David Hildenbrand <david@redhat.com>]

On 25.02.23 17:31, Peter Xu wrote:
> [not for merging, but for discussion; this is something I found when
>   looking at another issue on Chuang's optimization for migration downtime]
> 
> Summary: we tried to access memory_listeners, address_spaces, etc. in RCU
> way.  However we didn't implement them with RCU-safety. This patchset is
> trying to do that; at least making it closer.
> 
> NOTE!  It's doing it wrongly for now, so please feel free to see this as a
> thread to start discussing this problem, as in subject.
> 
> The core problem here is how to make sure memory listeners will be freed in
> RCU ways, per when unlinking them from the global memory_listeners list.

Can you elaborate why we would want to do that? Is there a real reason 
we cannot hold the BQL when unregistering a listener?

Or could we use any other, more fine-grained, lock to protect the memory 
listeners?

Naive me would think that any interactions between someone updating the 
memory listeners, and a listener getting removed, would require some 
careful synchronization (to not rip a notifier out while someone else 
notifies -- what is the still registered notifier supposed to do with 
notifications while it is already going away?), instead of doing it via RCU.

I'm all for using RCU if it improves performance and keeps things 
simple. If RCU is neither required for performance reason and 
overcomplicates the implementation, maybe using locking is the better 
choice.

TBH, so far I thought that any memory_listeners register/unregistering 
*requires* the BQL, and everything else is a BUG.

-- 
Thanks,

David / dhildenb