Date: Sat, 5 Mar 2016 18:32:54 -0500 From: Rich Felker <dalias@...c.org> To: Pedro Giffuni <pfg@...eBSD.org> Cc: musl@...ts.openwall.com Subject: Re: FreeBSD's Google Summer of Code 2016 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: > > 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. 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. In a worst case, futex can be emulated trivially by having wait and wake both be nops; then everything just spins at 100% cpu load. Of course this does not give very nice results. It's possible futex could be emulated better; certainly it can be within a single process where the futex wait queue could just be implemented in userspace, but emulating process-shared futexes effectively this way sounds nontrivial. Since the functionality is already there in the kernel, though, just exposing it sounds like a much better solution to me. Rich
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