Re: light weight user level semaphores

Alexander Viro
2001-04-19T17:33:49Z

On Thu, 19 Apr 2001, Linus Torvalds wrote:

>
>
> On Thu, 19 Apr 2001, Alexander Viro wrote:
> >
> > I certainly agree that introducing ioctl() in _any_ API is a shootable
> > offense. However, I wonder whether we really need any kernel changes
> > at all.
>
> I'd certainly be interested in seeing the pipe-based approach. Especially
> if you make the pipe allocation lazy. That isn'tr trivial (it needs to be
> done right with both up_failed() and down_failed() trying to allocate the
> pipe on contention and using an atomic cmpxchg-style setting if none
> existed before). It has the BIG advantage of working on old kernels, so
> that you don't need to have backwards compatibility cruft in the
> libraries.

Ehh... Non-lazy variant is just read() and write() as down_failed() and
up_wakeup() Lazy... How about

if (Lock <= 1)
goto must_open;
opened:
/* as in non-lazy case */

must_open:
pipe(fd);
lock decl Lock
jg lost_it /* Already seriously positive - clean up and go */
jl spin_and_lose
/* Lock went from 1 to 0 - go ahead */
reader = fd[0];
writer = fd[1];
Lock = MAX_INT;
goto opened;
spin_and_lose:
/* Won't take long - another guy got to do 3 memory writes */
while (Lock <= 0)
;
lost_it:
lock incl Lock
close(fd[0]);
close(fd[1]);
goto opened;

Al