On Tue, 2024-02-06 at 20:47 -0800, Sean Christopherson wrote:
I'm saying this:
When processing mmu_notifier invalidations for gpc caches, pre-check for overlap with the invalidation event while holding gpc->lock for read, and only take gpc->lock for write if the cache needs to be invalidated. Doing a pre-check without taking gpc->lock for write avoids unnecessarily contending the lock for unrelated invalidations, which is very beneficial for caches that are heavily used (but rarely subjected to mmu_notifier invalidations).
is much friendlier to readers than this:
Taking a write lock on a pfncache will be disruptive if the cache is heavily used (which only requires a read lock). Hence, in the MMU notifier callback, take read locks on caches to check for a match; only taking a write lock to actually perform an invalidation (after a another check).
That's a somewhat subjective observation. I actually find the latter to be far more succinct and obvious.
Actually... maybe I find yours harder because it isn't actually stating the situation as I understand it. You said "unrelated invalidation" in your first email, and "overlap with the invalidation event" in this one... neither of which makes sense to me because there is no *other* invalidation here.
We're only talking about the MMU notifier gratuitously taking the write lock on a GPC that it *isn't* going to invalidate (the common case), and that disrupting users which are trying to take the read lock on that GPC.