Re: light weight user level semaphores

[Abramo Bagnara <abramo@alsa-project.org>]

Alon Ziv wrote:
> 
> Hmm...
> I already started (long ago, and abandoned since due to lack of time :-( )
> down another path; I'd like to resurrect it...
> 
> My lightweight-semaphores were actually even simpler in userspace:
> * the userspace struct was just a signed count and a file handle.
> * Uncontended case is exactly like Linus' version (i.e., down() is decl +
> js, up() is incl()).
> * The contention syscall was (in my implementation) an ioctl on the FH; the
> FH was a special one, from a private syscall (although with the new VFS I'd
> have written it as just another specialized FS, or even referred into the
> SysVsem FS).
> 
> So, there is no chance for user corruption of kernel data (as it just ain't
> there...); and the contended-case cost is probably equivalent (VFS cost vs.
> validation).

This would also permit:
- to have poll()
- to use mmap() to obtain the userspace area

It would become something very near to sacred Unix dogmas ;-)

-- 
Abramo Bagnara                       mailto:abramo@alsa-project.org

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It sounds good!