Re: [RFC PATCH 0/5] locks: implement "filp-private" (aka UNPOSIX) locks

[Andreas Dilger]

On Fri, Oct 11, 2013 at 08:25:17AM -0400, Jeff Layton wrote:
> 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.

It isn't explicitly stated here, but will the POSIX and non-POSIX
locks interact with each other?  For example, if one process is using
the old POSIX semantics for lock release, but another process is using
the new non-POSIX semantics for lock release, will the two locks
otherwise behave as expected and conflict with each other if needed?

Cheers, Andreas

> 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.


Cheers, Andreas





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