At LSF this year, there was a discussion about the "wishlist" for
userland file servers. One of the things brought up was the goofy and
problematic behavior of POSIX locks when a file is closed. Boaz started
a thread on it here:
http://permalink.gmane.org/gmane.linux.file-systems/73364
Userland fileservers often need to maintain more than one open file
descriptor on a file. The POSIX spec says:
"All locks associated with a file for a given process shall be removed
when a file descriptor for that file is closed by that process or the
process holding that file descriptor terminates."
This is problematic since you can't close any file descriptor without
dropping all your POSIX locks. Most userland file servers therefore
end up opening the file with more access than is really necessary, and
keeping fd's open for longer than is necessary to work around this.
This patchset is a first stab at an approach to address this problem by
adding two new l_type values -- F_RDLCKP and F_WRLCKP (the 'P' is short
for "private" -- I'm open to changing that if you have a better
mnemonic).
For all intents and purposes these lock types act just like their
"non-P" counterpart. The difference is that they are only implicitly
released when the fd against which they were acquired is closed. As a
side effect, these locks cannot be merged with "non-P" locks since they
have different semantics on close.
I've given this patchset some very basic smoke testing and it seems to
do the right thing, but it is still pretty rough. If this looks
reasonable I'll plan to do some documentation updates and will take a
stab at trying to get these new lock types added to the POSIX spec (as
HCH recommended).
At this point, my main questions are:
1) does this look useful, particularly for fileserver implementors?
2) does this look OK API-wise? We could consider different "cmd" values
or even different syscalls, but I figured this makes it clearer that
"P" and "non-P" locks will still conflict with one another.
Jeff Layton (5):
locks: consolidate checks for compatible filp->f_mode values in setlk
handlers
locks: add definitions for F_RDLCKP and F_WRLCKP
locks: skip FL_FILP_PRIVATE locks on close unless we're closing the
correct filp
locks: handle merging of locks when FL_FILP_PRIVATE is set
locks: show private lock types in /proc/locks
fs/locks.c | 121 ++++++++++++++++++++++++++-------------
include/linux/fs.h | 1 +
include/uapi/asm-generic/fcntl.h | 9 +++
3 files changed, 91 insertions(+), 40 deletions(-)
--
1.8.3.1
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