Re: Proposal for a new lock type

[Florian Weimer <fw@deneb.enyo.de>]

* Matthew Wilcox:

> On Sun, Aug 15, 2010 at 05:06:58PM +0200, Florian Weimer wrote:
>> It seems that it might be useful to have a locally shared, globally
>> exclusive file lock.  "Local" is defined as "when the file is mapped
>> shared, memory accesses follow the architecture memory model across
>> threads and processes, and shared POSIX mutexes work".  This could be
>> used to make the new WAL code in SQLite 3.7 more foolproof, but there
>> are other applications which would benefit as well.
>
> This sounds like you're trying to fix a problem with NFS.  I think there
> are network filesystems which practice mmap coherency.

In the sense that I described?  This surprises me.

(I know that there is something called Distributed Shared Memory.  But
it's generally got different properties than a mapped file, AFAIK.)

>> It seems to me that this would not need changes to network file
>> systems because you can implement it by acquring a traditional
>> fcntl()-style exclusive lock on the network side, without creating a
>> corresponding local lock.
>
> fcntl locks are advisory ...

That's not a problem for this use case.  It's about making sure that
an SQLite-using application cannot accidentally access the same
database without proper locking because the mutexes are not visible
across a network file system.