Date: Tue, 13 Mar 2007 10:02:06 -0400 From: John Baldwin <jhb@freebsd.org> To: Julian Elischer <julian@elischer.org> Cc: Attilio Rao <attilio@freebsd.org>, cvs-src@freebsd.org, src-committers@freebsd.org, Pawel Jakub Dawidek <pjd@freebsd.org>, cvs-all@freebsd.org Subject: Re: cvs commit: src/share/man/man9 Makefile condvar.9 lock.9 mi_switch.9 mtx_pool.9 mutex.9 rwlock.9 sleep.9 sleepqueue.9 sx.9 thread_exit.9 src/sys/kern kern_synch.c src/sys/sys mutex.h rwlock.h sleepqueue.h sx.h systm.h Message-ID: <200703131002.07180.jhb@freebsd.org> In-Reply-To: <45F5E1F9.5090806@elischer.org> References: <200703092241.l29Mf2Ds062856@repoman.freebsd.org> <200703121618.41084.jhb@freebsd.org> <45F5E1F9.5090806@elischer.org>
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On Monday 12 March 2007 19:27, Julian Elischer wrote: > John Baldwin wrote: > > On Monday 12 March 2007 16:03, Pawel Jakub Dawidek wrote: > >> On Mon, Mar 12, 2007 at 03:35:21PM -0400, John Baldwin wrote: > >>> On Monday 12 March 2007 14:56, Pawel Jakub Dawidek wrote: > >>>> On Mon, Mar 12, 2007 at 11:16:23AM -0400, John Baldwin wrote: > >>>>> On Saturday 10 March 2007 15:52, Pawel Jakub Dawidek wrote: > >>>>>> What about something like this: > >>>>>> > >>>>>> #define cv_wait(cv, lock) do { > >>>>>> switch (LO_CLASSINDEX((struct lock_object *)(lock))) { > >>>>> The problem with a cast is you use type checking. Might as well do > > this: > >>>>> #define cv_wait(cv, lock) _cv_wait((cv), (struct lock_object *)(lock)) > >>>> This will skip type checking and my version only cast to provide type > >>>> checking, so when you pass some random variable it will give you an > >>>> error. > >>> Not really, you may pass some garbage and the LO_CLASSINDEX turns out to > > be a > >>> mutex. :) You only get a runtime error, not a compile-time one. > >>> Type-checking by the compiler is nice because you get compile-time errors. > >> I'll get compile-time error, because cv_wait_mtx() takes > >> 'struct condvar *' and 'struct mtx *' as arguments. So even if some > >> garbage returns 1, which turns out to be a mutex, call to cv_wait_mtx() > >> will generate compile-time error. > > > > Err, no, actually, yours will always give compile errors actually. Keep in > > mind that LO_CLASSINDEX() is a run-time check. This: > > > > #define cv_wait(cv, lock) do { > > switch (LO_CLASSINDEX((struct lock_object *)(lock))) { > > case 1: > > cv_wait_mtx(cv, lock); > > break; > > case 2: > > cv_wait_sx(cv, lock); > > break; > > case 3: > > cv_wait_rw(cv, lock); > > break; > > default: > > panic("Invalid lock."); > > } > > } while (0) > > > > Will try to pass 'lock' to three different functions, at least 2 of which will > > trigger compile errors. :) The kernel won't choose which one to run until > > runtime though. The key is that I want a compile error, not a panic(). :) > > I've been asking for awhile that for example spin and sleep mutexes should > be different types so that we could catch those problems at compile time. That is on my todo list actually. Stephan and I talked at BSDCan 06 about various alternative strategies for spin locks. -- John Baldwin
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