first patch for process-shared semaphore

[David Xu]

John Baldwin wrote:
> On Tuesday 22 December 2009 8:49:53 pm David Xu wrote:
>> This is my first attempt to make process-shared mutex work, this means
>> you can mmap(MAP_SHARED) a memory area, and put semaphore there,
>> or you can sem_open a named semaphore, and just use it between
>> processes, the named semaphore uses file system and mmap(), directory 
>> /tmp/.semaphore is used as IPC directory, any named semaphore
>> locates in the directory. old semaphore implementation still exists
>> to make it binary compatible, it uses symbol version.
>>
>> http://people.freebsd.org/~davidxu/patch/shared_semaphore_1.patch
> 
> I would suggest that you leave named semaphores as they currently exist and 
> follow this approach instead:
>  
> 1) Named semaphores use ksem_*() still.
> 2) sem_init/sem_destroy operate on UTMX-backed semaphores identical to the 
> ones used in the current libthr code.  The semid_t structure now becomes the 
> full structure that libthr currently allocates with a flag to indicate if it 
> is a "system" semaphore or otherwise.  The pshared flag passed to sem_init() 
> can be used to set the sharing properties of the UMTX.
> 3) All of sem_init/sem_destroy is just in libc.  Just move the libthr 
> implementation bits into libc.
> 

ksem base shared semaphore is slow because whenever you call
sem_wait(), it always enters kernel even if count is non-zero,
sem_post() also always enters kernel even if there is no waiter.
but the new implementation is as simple as just an atomic operation
in these cases, I know another competitor OS is doing things in
this way.