Re: [PATCH][RFC] Lightweight user-level semaphores

From: Matthew Kirkwood (
Date: Mon Jan 07 2002 - 17:03:36 EST

On Mon, 7 Jan 2002, Linus Torvalds wrote:

> > * It leaks. How were you going to refcount the kernel
> > portions? Could they be attached to the VM mapping?
> > Would a lockfs be too expensive?
> Yes, I was going to just attach to the vma,

Wouldn't that have to be an address_space, so separate maps
of the same object will use the same count? Or (not unlikely)
am I misunderstanding the way these structures are laid out?

> along with potentially also require a flag at mmap time (MAP_SEMAPHORE
> - some other unixes have something like it already) to tell the OS
> about the consistency issues that might come up on some architectures
> (on x86 it would be a no-op).


> > * It doesn't have a timeout. Is there something like a
> > down_timeout() available?
> Not as-is, but all the kernel infrastructure should be there in
> theory.

OK, thanks.

> > * I don't do the:
> >
> > if (kfs->user_address != fs)
> > goto bad_sem;
> >
> > because it doesn't seem to add anything, and prevents
> > putting these locks in a non-fixed file or SysV SHM
> > map.
> Fair enough. I think I suggested that just as another sanity check,
> and because some architectures _will_ require address issues (not
> necessarily total equality, but at least "modulo X equality").

Should being in the same place in the same page (though
possibly at a different address) should suffice for all


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