Re: [PATCH v2] cxl/region: Remove lock from memory notifier callback

[Davidlohr Bueso]

On Wed, 14 Aug 2024, Ira Weiny wrote:

> In testing Dynamic Capacity Device (DCD) support, a lockdep splat
> revealed an ABBA issue between the memory notifiers and the DCD extent
> processing code.[0]  Changing the lock ordering within DCD proved
> difficult because regions must be stable while searching for the proper
> region and then the device lock must be held to properly notify the DAX
> region driver of memory changes.
>
> Dan points out in the thread that notifiers should be able to trust that
> it is safe to access static data.  Region data is static once the device
> is realized and until it's destruction.  Thus it is better to manage the
> notifiers within the region driver.
>
> Remove the need for a lock by ensuring the notifiers are active only
> during the region's lifetime.

Agreed, this is better.

> Furthermore, remove cxl_region_nid() because resource can't be NULL
> while the region is stable.

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>