Re: [PATCH 3/7] cxl/region: Hold memdev lock during region poison injection/clear

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

Li Ming wrote:
> cxl_dpa_to_region() will require callers holding the given CXL memdev
> lock for endpoint validity checking in the following patch.

So, the justification for this is not that cxl_dpa_to_region() is going
to start enforcing lock holding, it is that cxl_dpa_to_region() has
expectations that ->endpoint remains valid for the duration of the call.

> To prepare
> it, region poison injection/clearing debugfs interfaces need to ensure
> the correct CXL memdev lock is held at the beginning.

My RFC [1] suggestion was "if there is a problem, here is locking
changes to fix it". That "if there is a problem" is what needed more thought
and time. Before we act on that RFC I want to see some plausible
analysis of how not holding this lock can cause a failure in theory
(better in practice), or how it makes the code hard to reason about.

In other words the justification is not "future patch adds a lockdep
assert so all code paths need to get more complicated". The
justification is identifying a theoretical race window and how the
locking solves it, and how the previous fix [2] did not handle all the
potential races.

[1]: http://lore.kernel.org/69813ac070f79_55fa1005c@dwillia2-mobl4.notmuch
[2]: 0066688dbcdc cxl/port: Hold port host lock during dport adding.