First of all, it is amazing that the author managed to analyze the patch in so much details, it probably is an effort comparable to the bug fix itself. Still I think the article is missing some bits. I would expect any core migration to require barriers (either implicit or explicit) on both the old and new core otherwise the process would risk seeing its own stores and loads out of order.

But in this case the barrier is predicated on the execution of some cache manipulation instruction, so I suspect things are more complicated. Maybe these specific cache manipulation instructions do not respect the usual architectural memory ordering and require some different set of barriers. Possibly they bypass cache coherence completely and require an actual flush of the cache. That is going to be very expensive and it make sense that it is only done only if the process was actually fiddling with these instructions. 'jmgao' else thread reported that tegra has coherency issues on migration, so it might be related.