Re: [PATCH v3] cxl: Add cxl_region_rwsem around commit_end write during decoder enumeration

[Dan Williams <dan.j.williams@intel.com>]

Davidlohr Bueso wrote:
> On Fri, 17 Nov 2023, Dave Jiang wrote:
> 
> >init_hdm_decoder() modifies port->commit_end without taking the
> >cxl_region_rwsem. An assert splat emitted by cxl_num_decoders_committed().
> >However looking at the code, it looks like the write version of the rwsem
> >needs to be taken due to the modification of commit_end. Wrap the write
> >version of the rwsem around reading and writing of commit_end.
> >
> >Fixes: 176baefb2eb5 ("cxl/hdm: Commit decoder state to hardware")
> >Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> 
> Uhmm but do we expect concurrency during the switch/port probing phase?

I answered that that detail here in response to Fan:

http://lore.kernel.org/r/654eb8ed72ced_46f0294c@dwillia2-mobl3.amr.corp.intel.com.notmuch

The takeaway is:

"I am in favor of adding the locking rather than depend on a subtle
side-effect of how CXL operates, and to avoid adding an unlocked version
of cxl_num_decoders_committed()"

I will add a note to the changelog to that effect, and drop Fixes: since
the lock is not strictly required for correctness in this path.