Re: FreeBSD's Google Summer of Code 2016

[Rich Felker <dalias@libc.org>]

On Sat, Mar 05, 2016 at 07:59:35PM -0500, Pedro Giffuni wrote:
> 
> 
> 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 [1]:
> >
> >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[1].
> 
> Luckily Ed, the developer that would be mentoring the project, knows
> this stuff better than I do.

OK, well let's see if there are any proposals and what ideas people
come up with.

Rich