semaphores between processes

[John Baldwin]

On Friday 23 October 2009 10:56:06 am Daniel Eischen wrote:
> On Fri, 23 Oct 2009, John Baldwin wrote:
> 
> > 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.
> 
> That's really not the way to go.  The structs really need
> to become public.

I was mostly suggesting it as a way to use something sooner since I expect it
will be a long while before anyone does the pthreads work.

-- 
John Baldwin