lock experienced contention a number of processes would race to acquire
lock when it was released. This problem resulted in a lot of CPU
load as well as locks being picked up out of order.
Unfortunately, a regression snuck in which allowed multiple threads
to pickup the same lock when -k was not used. This could occur when
multiple processes open a file descriptor to inode X (one process
will be blocked) and the file is unlinked on unlock (thereby removing
the directory entry allow another process to create a new directory
entry for the same file name and lock it).
This changes restores the old algorithm of: wait for the lock, then
acquire lock when we want to unlink the file on exit (specifically
when -k is not used) and keeps the new algorithm for when -k is used,
which yields fairness and improved performance.
Also, update the man page to inform users that if lockf(1) is being
used to facilitate concurrency between a number of processes, it
is recommended that -k be used to reduce CPU load and yeld
fairness with regard to lock ordering.
Collaborated with: jdp
PR: bin/114341
PR: bin/116543
PR: bin/111101
MFC after: 1 week
11b9e24e5b