074ad60a4c
At the time opt-in was introduced adding yourself as a writer was esrializing across the mount point. Nowadays it is fully per-cpu, the only impact being a small single-threaded hit on top of what's there right now. Vast majority of the overhead stems from the call to VOP_GETWRITEMOUNT which has is done regardless. Should someone want to microoptimize this single-threaded they can coalesce looking the mount up with adding a write to it.
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