Date: Sat, 5 Mar 2016 19:59:35 -0500 From: Pedro Giffuni <pfg@...eBSD.org> To: Rich Felker <dalias@...c.org> Cc: musl@...ts.openwall.com Subject: Re: FreeBSD's Google Summer of Code 2016 On 03/05/16 19:25, Rich Felker wrote: > On Sat, Mar 05, 2016 at 07:14:34PM -0500, Pedro Giffuni wrote: >> >> >> On 03/05/16 18:32, Rich Felker wrote: >>> On Sat, Mar 05, 2016 at 05:41:25PM -0500, Pedro Giffuni wrote: >>>> First of all, great to hear there is interest on the musl side too. >>>> >>>> I think the biggest precedent of porting linux-oriented C libraries >>>> came from Debian's kFreeBSD. We accomodated a little by for them >>>> by defining __FreeBSD_kernel__ in sys/param.h. >>>> >>>> While using the optional linux-abi futex in FreeBSD could be an option, >>>> it is not really the cleanest option. The Debian guys did a port of >>>> NPTL using regular pthreads: >>>> >> >> Of course I ahould have meant "based on regular FreeBSD kernel services". >> >>>> http://thread.gmane.org/gmane.linux.debian.ports.bsd/11702 >>>> >>>> I am certain this will require more research but it would be useful >>>> for other ports as well. >>> >> >> We could ask Petr Salinger for the details, but I am pretty sure >> FreeBSD has the required functionality natively. >> >>> Glibc/NPTL has a lot of what I'd call "gratuitous abstraction" (like >>> the lll stuff) in their pthread primitives which makes this >>> "possible". I call it gratuitous because it's really really hard to >>> achieve correct implementations of the pthread sync primitives that >>> don't have serious corner-case bugs, and it's unlikely that their >>> abstractions actually suffice to make correct alternate >>> implementations. >>> >>> musl does not have any such abstraction. We require a compare-and-swap >>> operation or equivalent on which arbitrary atomic operations can be >>> constructed, a futex or equivalent operation that's roughly >>> while(*addr==expected) sleep(), and implement all the sync primitives >>> just once on top of these. >>> >> >> I am not a threading expert (or even a CS guy), but it sounds like >> mutex(9) with condvar(9) would do : > > No, they don't satisfy the needs of musl; they have their own > additional storage requirements and are probably not AS-safe. It might > be possible to use them to implement a userspace-emulated futex queue > (only if they are AS-safe), but I don't see a way to extend that to > the process-shared case. > OK, it looks like sema(9) may be nearer (and also simpler but slower). For the process-shared case libthr(2) uses the stuff in sys/utmx.h, shich should be looked at but it is not documented. Luckily Ed, the developer that would be mentoring the project, knows this stuff better than I do. Pedro.  http://fxr.watson.org/fxr/source/sys/umtx.h?v=FREEBSD8
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.