Date: Fri, 23 Oct 2009 10:56:06 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-hackers@freebsd.org, Andrew Gallatin <gallatin@cs.duke.edu>, Christian Bell <christian@myri.com> Subject: Re: semaphores between processes Message-ID: <Pine.GSO.4.64.0910231055270.16088@sea.ntplx.net> In-Reply-To: <200910230802.49873.jhb@freebsd.org> References: <4AE0BBAB.3040807@cs.duke.edu> <4AE0C995.5060303@cs.duke.edu> <Pine.GSO.4.64.0910221715330.11443@sea.ntplx.net> <200910230802.49873.jhb@freebsd.org>
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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
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