semaphores between processes

[Daniel Eischen]

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.

-- 
DE