This extension allows to use F_UNLCK on query, which currently returns EINVAL. Instead it can be used to query the locks on a particular fd - something that is not currently possible. The basic idea is that on F_OFD_GETLK, F_UNLCK would "conflict" with (or query) any types of the lock on the same fd, and ignore any locks on other fds.
Use-cases:
1. CRIU-alike scenario when you want to read the locking info from an fd for the later reconstruction. This can now be done by setting l_start and l_len to 0 to cover entire file range, and do F_OFD_GETLK. In the loop you need to advance l_start past the returned lock ranges, to eventually collect all locked ranges.
2. Implementing the lock checking/enforcing policy. Say you want to implement an "auditor" module in your program, that checks that the I/O is done only after the proper locking is applied on a file region. In this case you need to know if the particular region is locked on that fd, and if so - with what type of the lock. If you would do that currently (without this extension) then you can only check for the write locks, and for that you need to probe the lock on your fd and then open the same file via another fd and probe there. That way you can identify the write lock on a particular fd, but such trick is non-atomic and complex. As for finding out the read lock on a particular fd - impossible. This extension allows to do such queries without any extra efforts.
3. Implementing the mandatory locking policy. Suppose you want to make a policy where the write lock inhibits any unlocked readers and writers. Currently you need to check if the write lock is present on some other fd, and if it is not there - allow the I/O operation. But because the write lock can appear at any moment, you need to do that under some global lock, which can be released only when the I/O operation is finished. With the proposed extension you can instead just check the write lock on your own fd first, and if it is there - allow the I/O operation on that fd without using any global lock. Only if there is no write lock on this fd, then you need to take global lock and check for a write lock on other fds.
The second patch adds a test-case for OFD locks. It tests both the generic things and the proposed extension.
The third patch is a proposed man page update for fcntl(2) (not for the linux source tree)
Changes in v2: - Dropped the l_pid extension patch and updated test-case accordingly.
Stas Sergeev (2): fs/locks: F_UNLCK extension for F_OFD_GETLK selftests: add OFD lock tests
fs/locks.c | 23 +++- tools/testing/selftests/locking/Makefile | 2 + tools/testing/selftests/locking/ofdlocks.c | 132 +++++++++++++++++++++ 3 files changed, 154 insertions(+), 3 deletions(-) create mode 100644 tools/testing/selftests/locking/ofdlocks.c
CC: Jeff Layton jlayton@kernel.org CC: Chuck Lever chuck.lever@oracle.com CC: Alexander Viro viro@zeniv.linux.org.uk CC: Christian Brauner brauner@kernel.org CC: linux-fsdevel@vger.kernel.org CC: linux-kernel@vger.kernel.org CC: Shuah Khan shuah@kernel.org CC: linux-kselftest@vger.kernel.org CC: linux-api@vger.kernel.org