Date: Mon, 7 Mar 2016 09:32:17 +0100 From: Felix Fietkau <nbd@...nwrt.org> To: musl@...ts.openwall.com, Pedro Giffuni <pfg@...eBSD.org> Subject: Re: FreeBSD's Google Summer of Code 2016 On 2016-03-06 01:25, 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. What about umtx? It's supposed to be just like linux futex. - Felix
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