The lock is taken all the time and as a regular read-write lock
avoidably serves as a mount point-wide contention point.
This forward ports FreeBSD revision r357322.
To quote aforementioned commit:
Sample result doing an incremental -j 40 build:
before: 173.30s user 458.97s system 2595% cpu 24.358 total
after: 168.58s user 254.92s system 2211% cpu 19.147 total
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes#10896
29bc31f62f