By default, lockf(1) opens its lock file O_RDONLY|O_EXLOCK. On NFS, if the
file already exists, this is split into opening the file read-only and then
requesting an exclusive lock -- and the second step fails because NFS does
not permit exclusive locking on files which are opened read-only.
The new -w option changes the open flags to O_WRONLY|O_EXLOCK, allowing it
to work on NFS -- at the cost of not working if the file cannot be opened
for writing.
(Whether the traditional BSD behaviour of allowing exclusive locks to be
obtained on a file which cannot be opened for writing is a good idea is
perhaps questionable since it may allow less-privileged users to perform
a local denial of service; however this behaviour has been present for a
long time and changing it now seems like it would cause problems.)
Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D26005
exist
Apply EX_UNAVAILABLE patch part from PR 170775 to match the documentation.
Checked with a command from PR 210770:
lockf -n /tmp/doesnotexist echo; echo $?
PR: 210770
MFC after: 1 week
Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.
The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.
No functional change intended.
This is easy to confuse with the actual exit status of the program.
Instead exit with EX_SOFTWARE if the command doesn't exit normally.
MFC after: 1 month
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
Add a flags argument to wait_for_lock so that O_NONBLOCK can be
passed to open if a user doesn't want the open to sleep until the
lock becomes available.
Submitted by: Amir Shalem (partially modified)
for mutual exclusion:
A brief description of the problem:
1) Proc A picks up non-blocking lock on file X
2) Proc B attempts to pickup lock, fails then waits
3) Proc C attempts to pickup lock, fails then waits
4) Proc A releases lock
5) Proc B acquires lock, release it to pickup a non-blocking version
6) Proc C acquires lock, release it to pickup a non-blocking version
7) Both process B and C race each other to pickup lock again
This occurs mainly because the processes do not keep the lock after they have
been waiting on it. They drop it, attempt to re-acquire it. (They use the wait
to notify when the lock has become available then race to pick it up). This
results in additional CPU utilization during the race, and can also result
in processes picking locks up out of order.
This change attempts to correct this problem by eliminating the test/acquire
race and having the operating system handle it.
Reported by: kris
Tested by: kris
MFC after: 1 week
This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
return EX_TEMPFAIL if the file was already locked. This makes it easier
to distinguish between lock collisions and failures within the command
being executed.
Also, don't complain if the unlink() fails in the cleanup handler. It
doesn't matter anyway, and it obscured the exit status returned from
the command that was executed.
