Dropping cache didn’t drop cache
I know that Linus hates unit tests, but these kinds of scenarios are perfect for regression tests. The investigation of the issue took so long, that you'd really want to spend those extra few hours writing the test to save yourself investigation in the future. The patch doesn't include any comment about a race condition in the actual code, so let's assume that all the knowledge is more-or-less lost and forgotten in 12 months or so.