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    SubjectRe: light weight user level semaphores


    On Thu, 19 Apr 2001, Abramo Bagnara wrote:

    > 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 ;-)

    I suspect that simple pipe with would be sufficient to handle contention
    case - nothing fancy needed (read when you need to block, write upon up()
    when you have contenders)

    Would something along the lines of (inline as needed, etc.)

    down:
    lock decl count
    js __down_failed
    down_done:
    ret

    up:
    lock incl count
    jle __up_waking
    up_done:
    ret

    __down_failed:
    call down_failed
    jmp down_done
    __up_waking:
    call up_waking
    jmp up_done

    down_failed()
    {
    read(pipe_fd, &dummy, 1);
    }

    up_waking()
    {
    write(pipe_fd, &dummy, 1);
    }

    be enough?
    Al

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