Re: FreeBSD's Google Summer of Code 2016

[Rich Felker <dalias@libc.org>]

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.

Rich