Date: Fri, 23 Oct 2009 11:47:36 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-hackers@freebsd.org, Christian Bell <christian@myri.com> Subject: Re: semaphores between processes Message-ID: <Pine.GSO.4.64.0910231142330.16269@sea.ntplx.net> In-Reply-To: <4AE1CE31.1090206@cs.duke.edu> 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> <Pine.GSO.4.64.0910231055270.16088@sea.ntplx.net> <4AE1CE31.1090206@cs.duke.edu>
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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 >>>>>>> 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. >> > > 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. -- DE
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