Re: light weight user level semaphores

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

Alexander Viro wrote:
> 
> 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?

There is something wonderful in this simple solution.

However I've a few doubts:
- choice policy for thread to wake is not selectable
- we separate shared memory area from file descriptor
- the implementation of down_try has neither been discussed nor
excluded, but I don't see how to implement it

The implementation of a specific filesystem seems to me more flexyble.

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

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It sounds good!