Date: Wed, 10 Feb 2010 13:22:15 -0500 (EST) From: Daniel Eischen <eischen@vigrid.com> To: Randall Stewart <rrs@lakerest.net> Cc: threads@freebsd.org Subject: Re: Thinking about kqueue's and pthread_cond_wait Message-ID: <Pine.GSO.4.64.1002101317210.14115@sea.ntplx.net> In-Reply-To: <07AA24BB-DA26-406A-B24F-59E0CB36FEBE@lakerest.net> References: <3581A86D-9C9C-4E08-9AD3-CD550B180CED@lakerest.net> <Pine.GSO.4.64.1002101202060.13656@sea.ntplx.net> <3CF3033E-FD13-405B-9DC6-DDE9DF4FBF37@lakerest.net> <Pine.GSO.4.64.1002101232240.13876@sea.ntplx.net> <07AA24BB-DA26-406A-B24F-59E0CB36FEBE@lakerest.net>
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On Wed, 10 Feb 2010, Randall Stewart wrote: > > On Feb 10, 2010, at 9:46 AM, Daniel Eischen wrote: > >> On Wed, 10 Feb 2010, Randall Stewart wrote: >> >>> >>> On Feb 10, 2010, at 9:04 AM, Daniel Eischen wrote: >>> >>>> On Wed, 10 Feb 2010, Randall Stewart wrote: >>>>> All: >>>>> I have once again come around to thinking about joining pthread cond >>>>> waits and >>>>> kqueue's. >>>>> After thinking about it, I think its doable.. with something like a: >>>>> pthread_cond_wait_kqueue_np(kev, cond, mtx, ucontext) >>>>> Then you can use kev inside a kqueue i.e. >>>>> ret = kevent(kq, kev, 1, outkev, 1, NULL); >>>>> Now when you saw the event: >>>>> if (kev.filter == EVFILT_UMTX){ /* not sure about the name here */ >>>>> pthread_kqueue_cond_wait_ret_np(kev, cond, mtx, ucontext) >>>>> do_user_action(cond,mtx, ucontext); >>>>> } >>>>> Which would fill in the cond/mtx and ucontext for the user. >>>>> Now does this sound useful to anyone.. i.e. should I spend the time >>>>> making it work? >>>>> The only down side to this is that it would have to allocate memory so >>>>> one would need to do a: >>>>> pthread_kqueue_cond_wait_free_np(kev) >>>>> After you were done.. and I think it would be best for this to >>>>> be a ONE_SHOT.. i.e. you have to re-arm it if the event happens... >>>>> Of course until you free it that can be as simple as passing the kev >>>>> back down again (i.e. no pthread_cond_wait_kqueue_np() needed). >>>>> Comments? Thoughts? i.e. especially is it worthwhile doing? >>>> Please don't mess with the pthread_ API like that :-) If you >>>> really want to munge them together, see my email to you a few >>>> weeks ago last time you brought it up.' >>> >>> If I remember right your email was basically don't do it... I will >>> go dig through the archives and re-read it all. >> >> No, it was to add an interface or two to the kqueue/kevent API, not >> to modify the pthread_ API (which shouldn't know anything about >> kqueues). >> >> I really think the OS is already given us the tools we need to >> do the job simply enough. You can easily use a pipe, socketpair, >> or EVFILT_SIGNAL to wakeup a thread stuck in kevent(). You can >> additionally use a mutex to protect data shared between thread >> waiting in kevent() and other threads. >> >> I don't see what problem this is trying to solve and I think >> whatever solution you come up with involving mutexes/CVs is >> not going to be any simpler and may even be more complex and >> messy. Mutexes and CVs are userland library thingies, not >> kernel entities. Yes, the umtx is a kernel entity, but it >> alone does not give you mutexes and CVs. So when you want >> to mix kqueues and mutexes/CVs, you are involving another >> userland library and this is what makes it messy. > > You suggested: > > kq = kqueue(); > kq_obj = kq_create(kq, ...); > kq_lock(&kq_obj); > while (!P) { > /* Atomically unlocks kq_obj and blocks. */ > nevents = kq_wait(&kq_obj, ...); > /* When you wakeup, kq_obj is locked again. */ > } > do_work(); > > > But that does not satisfy anything but the condition variable. What > would be nice is Yes, it does. You (the implementors of kq_create()) are suppose to maintain the mutex and/or CV inside the kq_obj thingie and hide it from the user of the interface. Whatever you need to lock and wait is embedded in kq_obj. You will have to do some of the same things that libthr does with umtx in pthread_mutex_{lock,unlock} and pthread_cond_{wait,signal,broadcast}. -- DE
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