Re: [PATCH] Lightweight userspace semaphores...

[Hubertus Franke]

On Mon, Feb 25, 2002 at 11:32:40AM -0500, Benjamin LaHaise wrote:
> On Mon, Feb 25, 2002 at 04:39:56PM +0000, Alan Cox wrote:
> > _alloca
> > mmap
> > 
> > Still fits on the stack 8)
> 
> Are we sure that forcing semaphore overhead to the size of a page is a 
> good idea?  I'd much rather see a sleep/wakeup mechanism akin to wait 
> queues be exported by the kernel so that userspace can implement a rich 
> set of locking functions on top of that in whatever shared memory is 
> being used.
> 
> 		-ben
> -

Amen, I agree Ben. As I indicated in my previous note, one can
implement various versions of spinning, starvation, non-starvation locks
based on the appropriateness for a given app and scenario.
For instance the multiple reader/single writer requires 2 queues.
If as Ben stated something similar to the SysV implementation is desired
where a single lock holds multiple waiting queues, that should be straight
forward to implement. Waiting queues could be allocated on demand as well.

I'd like to see an implementation that facilitate that.
My implementation separates the state to the user level and the 
waiting to the kernel level. There are race conditions that need to 
be resolved with respect to wakeup. They can be all encoded into
the atomic word maintained in shared memory in user space.

For more complex locks I'd like to have compare_and_swap instructions.
As I stated, I have implemented some of the more complicated locks
(spinning, convoy avoidance, etc.) and they have all passed some rigorous
stress test.

As for allocation on the stack. If indeed there are kernel objects
associated with the address, they need to be cleared upon exit from
the issueing subroutine (at least in my implementation).

At this point, could be go through and delineate some of the requirements
first.
E.g.  (a) filedescriptors vs. vaddr
      (b) explicit vs. implicite allocation  
      (c) system call interface vs. device driver
      (d) state management in user space only or in kernel as well
	        i.e. how many are waiting, how many are woken up.
      (e) semaphores only or multiple queues
      (f) protection through an exported handle with some MAGIC or
          through virtual memory access rights
      (g) persistence on mmap or not 

Here is my point of view:
(a) vaddr 
(b) implicite
(c) syscall
(d) user only
(e) multiple queues
(f) virtual memory access rights.
(g) persistent  (if you don't want persistence you remove the underlying object)
	
I requested some input on my original message a couple of weeks regarding 
these points (but only got one on (b)).

Could everybody respond to (a)-(f) for a show of hands.
Could we also consolidate some pointers of the various implementations
that are out there and then see what the pluses and minuses of the various
implementations are  and how they score against (a)-(f).

-- Hubertus Franke