Date: Tue, 31 Mar 2009 10:18:30 +0800 From: David Xu <davidxu@freebsd.org> To: Daniel Eischen <deischen@freebsd.org> Cc: threads@freebsd.org Subject: Re: A mutex for inter-process ;-) Message-ID: <49D17D76.5060309@freebsd.org> In-Reply-To: <Pine.GSO.4.64.0903301935300.2318@sea.ntplx.net> References: <7D4F6788-0F12-4863-9635-7FADA9115D16@lakerest.net> <Pine.GSO.4.64.0903301719000.2318@sea.ntplx.net> <AC6F7359-28D5-4F92-93AF-43B6AF86FC01@lakerest.net> <Pine.GSO.4.64.0903301935300.2318@sea.ntplx.net>
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Daniel Eischen wrote: > On Mon, 30 Mar 2009, Randall Stewart wrote: > >> >> On Mar 30, 2009, at 5:22 PM, Daniel Eischen wrote: >> >>> On Mon, 30 Mar 2009, Randall Stewart wrote: >>> >>>> Hi all: >>>> >>>> I have recently written a small set of routines that allow >>>> two process to have a "mutex" between them.. actually it allows >>>> all of the threads in any set of processes to have mutexes between >>>> themselves ;-) >>>> >>>> Anyway it seems to be working fairly well.. I still have to write a >>>> man page >>>> for it (documentation always last).. and eventually I would like to >>>> port in >>>> some of the WITNESS type features since the mutex's have names.. >>>> >>>> I probably should also think about scaling it up a bit.. right now >>>> its really >>>> more for a small scale (100 or less mutexes)... >>>> >>>> Who should I talk to about getting this in... having it reviewed >>>> etc. I think >>>> it belongs in libthr since it really needs the tid of the pthreads >>>> from the >>>> pthread_t type... and for now I have a horrible hack in to get it ;-) >>> >>> The real way to do this is to support PTHREAD_PROCESS_SHARED >>> mutexes within our normal mutex, and to change our current >>> mutex (and cv) types to be structs instead of pointers. >>> The current API, other than the type change, shouldn't >>> change at all. >> >> >> So how do you propose to name the mutex's so that two disparate >> process can locate the same mutex? > > They are placed in shared memory, according to POSIX. > >> I don't see how a pthread_mutex can suffice... we need more than >> just the current mutex... >> >> What am I missing? > > As far as I know, David Xu implemented the kernel hooks > for umtx (the underlying mutex in pthread mutex) to be > shared. As soon as you can place the entire userland > pthread_mutex_t struct in shared memory, it should all > just work (with probably some trivial changes in libthr). > The harder part is versioning all the symbols that > currently think pthread_mutex_t, pthread_cond_t, etc, > are pointers, and defining the structs with enough > foresight so that it is unlikely we have to modify > them in the future (causing a future ABI breakage), > and also aligning them nicely for the various archs. > > You should really look at how POSIX defines process > shared mutex, cvs, etc. See: > > pthread_barrierattr_[gs]etpshared() > pthread_condattr_[gs]etpshared() > pthread_mutexattr_[gs]etpshared() > pthread_wrlockattr_[gs]etsphared() > > You can use this as a starting point: > > http://www.opengroup.org/onlinepubs/009695399/ > > http://www.opengroup.org/onlinepubs/009695399/functions/pthread_barrierattr_setpshared.html > > > http://www.opengroup.org/onlinepubs/009695399/functions/pthread_condattr_setpshared.html > > > http://www.opengroup.org/onlinepubs/009695399/functions/pthread_mutexattr_setpshared.html > > > http://www.opengroup.org/onlinepubs/009695399/functions/pthread_rwlockattr_setpshared.html > > You are right. umtx is ready for process-shared mutex, condition variable and rwlock. We are blocked by our pthread_mutex_t and pthread_cond_t definitions which are pointers, mmap()ing it into shared memory and calling pthread API will not work correctly, they should be defined as a block of memory. Recent POSIX standard introduces robust mutex type which can detects mutex owner's death, but in theory, shared memory model will never be robust. Regards, David Xuhome | help
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