Date: Sat, 5 Mar 2016 19:25:47 -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 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. Rich
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