semaphores between processes

[John Baldwin]

On Thursday 22 October 2009 5:17:07 pm Daniel Eischen wrote:
> On Thu, 22 Oct 2009, Andrew Gallatin wrote:
> 
> > Daniel Eischen wrote:
> >> On Thu, 22 Oct 2009, Andrew Gallatin wrote:
> >> 
> >>> Hi,
> >>> 
> >>> We're designing some software which has to lock access to
> >>> shared memory pages between several processes, and has to
> >>> run on Linux, Solaris, and FreeBSD.  We were planning to
> >>> have the lock be a pthread_mutex_t residing in the
> >>> shared memory page.  This works well on Linux and Solaris,
> >>> but FreeBSD (at least 7-stable) does not support
> >>> PTHREAD_PROCESS_SHARED mutexes.
> >>> 
> >>> We then moved on to posix semaphores.  Using sem_wait/sem_post
> >>> with the sem_t residing in a shared page seems to work on
> >>> all 3 platforms.  However, the FreeBSD (7-stable) man page
> >>> for sem_init(3) has this scary text regarding the pshared
> >>> value:
> >>>
> >>>     The sem_init() function initializes the unnamed semaphore pointed to 
> >>> by
> >>>     sem to have the value value.  A non-zero value for pshared specifies 
a
> >>>     shared semaphore that can be used by multiple processes, which this
> >>>     implementation is not capable of.
> >>> 
> >>> Is this text obsolete?  Or is my test just "getting lucky"?
> >> 
> >> I think you're getting lucky.
> >
> > Yes, after playing with the code some, I now see that. :(
> >
> >>> Is there recommended way to do this?
> >> 
> >> I believe the only way to do this is with SYSV semaphores
> >> (semop, semget, semctl).  Unfortunately, these are not as
> >> easy to use, IMHO.
> >
> > Yes, they are pretty ugly, and we were hoping to avoid them.
> > Are there any plans to support either PTHREAD_PROCESS_SHARED
> > mutexes, or pshared posix semaphores in FreeBSD?
> 
> It's planned, just not (yet) being actively worked on.
> It's a API change mostly, and then adding in all the
> compat hooks so we don't break ABI.

There are also an alternate set of patches on threads@ to allow just shared 
semaphores I think w/o the changes to the pthread types.  I can't recall 
exactly what they did, but I think rrs@ was playing with using umtx directly 
to implement some sort of process-shared primitive.

-- 
John Baldwin