Fixing Posix semaphores

[Joe Kelsey]

On Mon, 2004-12-13 at 18:34 -0500, David Schultz wrote:
> On Mon, Dec 13, 2004, Joe Kelsey wrote:
> > On Mon, 2004-12-13 at 14:21 -0800, Julian Elischer wrote:
> > > 
> > > Joe Kelsey wrote:
> > > 
> > > >I have a desire to fix posix semaphores in at least 5.3.  The current
> > > >implementation doesn't actually follow the "spirit" of the standard,
> > > >even though it technically qualifies in a somewhat degraded sense.  I
> > > >refer to the fact that the current implementation treats posix
> > > >semaphores as completely contained inside the kernel and essentially
> > > >divorced from the filesystem.  The true "spirit" of the standard places
> > > >the semaphores directly in the file system, similar to named pipes.
> > > >However the current implementation treats the supplied "name" as a
> > > >14-character identifier, required to begin with a slash and contain no
> > > >other slashes.  Pretty weak.
> > > >
> > > >Well, in order to fix this, we need to add file system code and come up
> > > >with a new type.  I currently have some time to spend on something like
> > > >this and am willing to put in whatever effort it takes.  Does anyone
> > > >want to add their own ideas or requirements?
> > > >
> > > >I currently run 5.3, but I suppose I could think about running current
> > > >at some point in the future.
> > > >
> > > 
> > > I don't think that the spirit is to do what you suggest.
> > > I have always interpretted it to be a separate namespace.
> > > does the posix "mknod" definition mention how to make a semaphore?
> > 
> > POSIX does not define or allow use of mknod to create a named semaphore.
> > Only sem_open() can create a named semaphore.  The "spirit", as
> > implemented in other OS', clearly indicates the use of file system
> > names, not the restricted 14-character name used by FreeBSD.  For
> > instance, Solaris uses file system names.
> 
> Err, I'm pretty sure Solaris uses a separate namespace for
> semaphores, and I think Linux does the same.  That's not to say
> that I'm opposed to this idea.  However, the implementation you
> propose, while aesthetically pleasing, is likely to be much slower
> and take a good deal of effort.  Moreover, it doesn't seem that it
> would provide any significant additional functionality.

The Solaris documentation specifically says that sem_open uses file
system namespace.

The whole point of POSIX semaphores is that there are TWO different ways
to create semaphores.  sem_init() creates unnamed semaphores.  sem_open
creates named semaphores.  Therefore, you choose at creation time
whether you want speed or interoperability.  The speed penalty only
applies to creation/open.  Once you have the semaphore, the rest is the
same.

The whole point of having a file system name is to allow for unplanned
sharing in a complex way.  Using stupid 14-character names does not
allow for the use of sub-directories and sym-links and so on to really
take advantage of the complex name space.

Of course, if no one really wants to see this, then I guess I can just
hide over in my little corner since *I* want to see this happen.

/Joe