semaphores between processes

Daniel Eischen deischen at
Fri Oct 23 15:47:40 UTC 2009

On Fri, 23 Oct 2009, Andrew Gallatin wrote:

> 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
>>>>>>> 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.
> It would be great if they were, but that discussion was 6 months
> ago, and nothing seems to have happened.  Plus we need to support
> at least 7.X and probably 6, so any changes here might not even
> help us.
> What is wrong  with just using umtx directly?  It seems to do
> exactly what we need.

Because you can't do anything more than use umtx directly,
like check for mutex types and return appropriate error
codes.  Just look at other implementations - Solaris,
Linux, all have their pthread_*_t as public structs.


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